Isolation of highly enriched primary human microglia for functional studies.

Microglia, the resident macrophages of the central nervous system play vital roles in brain homeostasis through clearance of pathogenic material. Microglia are also implicated in neurological disorders through uncontrolled activation and inflammatory responses. To date, the vast majority of microglial studies have been performed using rodent models. Human microglia differ from rodent counterparts in several aspects including their response to pharmacological substances and their inflammatory secretions. Such differences highlight the need for studies on primary adult human brain microglia and methods to isolate them are therefore required. Our procedure generates microglial cultures of >95% purity from both biopsy and autopsy human brain tissue using a very simple media-based culture procedure that takes advantage of the adherent properties of these cells. Microglia obtained in this manner can be utilised for research within a week. Isolated microglia demonstrate phagocytic ability and respond to inflammatory stimuli and their purity makes them suitable for numerous other forms of in vitro studies, including secretome and transcriptome analysis. Furthermore, this protocol allows for the simultaneous isolation of neural precursor cells during the microglial isolation procedure. As human brain tissue is such a precious and valuable resource the simultaneous isolation of multiple cell types is highly beneficial.

Isolation and culture of adult human microglia within mixed glial cultures for functional experimentation and high-content analysis.

Microglia are thought to be involved in diseases of the adult human brain as well as normal aging processes. While neonatal and rodent microglia are often used in studies investigating microglial function, there are important differences between rodent microglia and their adult human counterparts. Human brain tissue provides a unique and valuable tool for microglial cell and molecular biology. Routine protocols can now enable use of this culture method in many laboratories. Detailed protocols and advice for culture of human brain microglia are provided here. We demonstrate the protocol for culturing human adult microglia within a mixed glial culture and use a phagocytosis assay as an example of the functional studies possible with these cells as well as a high-content analysis method of quantification.

M-CSF increases proliferation and phagocytosis while modulating receptor and transcription factor expression in adult human microglia.

BACKGROUND: Microglia are the primary immune cells of the brain whose phenotype largely depends on their surrounding micro-environment. Microglia respond to a multitude of soluble molecules produced by a variety of brain cells. Macrophage colony-stimulating factor (M-CSF) is a cytokine found in the brain whose receptor is expressed by microglia. Previous studies suggest a critical role for M-CSF in brain development and normal functioning as well as in several disease processes involving neuroinflammation. METHODS: Using biopsy tissue from patients with intractable temporal epilepsy and autopsy tissue, we cultured primary adult human microglia to investigate their response to M-CSF. Mixed glial cultures were treated with 25 ng/ml M-CSF for 96 hours. Proliferation and phagocytosis assays, and high through-put immunocytochemistry, microscopy and image analysis were performed to investigate microglial phenotype and function. RESULTS: We found that the phenotype of primary adult human microglia was markedly changed following exposure to M-CSF. A greater number of microglia were present in the M-CSF- treated cultures as the percentage of proliferating (BrdU and Ki67-positive) microglia was greatly increased. A number of changes in protein expression occurred following M-CSF treatment, including increased transcription factors PU.1 and C/EBPß, increased DAP12 adaptor protein, increased M-CSF receptor (CSF-1R) and IGF-1 receptor, and reduced HLA-DP, DQ, DR antigen presentation protein. Furthermore, a distinct morphological change was observed with elongation of microglial processes. These changes in phenotype were accompanied by a functional increase in phagocytosis of Aß1-42 peptide. CONCLUSIONS: We show here that the cytokine M-CSF dramatically influences the phenotype of adult human microglia. These results pave the way for future investigation of M-CSF-related targets for human therapeutic benefit.

The transcription factor PU.1 is critical for viability and function of human brain microglia.

Microglia are the predominant resident immune cells of the brain and can assume a range of phenotypes. They are critical for normal brain development and function but can also contribute to many disease processes. Although they are widely studied, the transcriptional control of microglial phenotype and activation requires further research. PU.1 is a key myeloid transcription factor expressed by peripheral macrophages and rodent microglia. In this article, we report the presence of PU.1 specifically in microglia of the adult human brain and we examine its functional role in primary human microglia. Using siRNA, we achieved substantial PU.1 protein knock-down in vitro. By assessing a range of characteristic microglial proteins we found decreased viability of adult human microglia with reduced PU.1 protein expression. This observation was confirmed with PU.1 antisense DNA oligonucleotides. An important function of microglia is to clear debris by phagocytosis. We assessed the impact of loss of PU.1 on microglial phagocytosis and show that PU.1 siRNA reduces the ability of adult human microglia to phagocytose amyloid-beta1-42 peptide. These results show that PU.1 controls human microglial viability and function and suggest PU.1 as a molecular target for manipulation of human microglial phenotype.

Primary mixed glial cultures from fetal ovine forebrain are a valid model of inflammation-mediated white matter injury.

Astrocytes, microglial cells and oligodendrocytes (OLs) have been employed separately in vitro to assess cellular pathways following a variety of stimuli. Mixed glial cell cultures, however, have not been utilized to the same extent, despite the observed discrepancy in outcomes resulting from cell-to-cell contact of different glia in culture. Our objective was to standardize and morphologically characterize a primary culture of preterm ovine glial cells in order to attain a relevant in vitro model to assess the intracellular effects of infection and inflammation. This would provide a high-throughput model necessary for in-depth studies on the various pathophysiological mechanisms of white matter injury (WMI), which may occur in the preterm infant as a consequence of maternal infection or the fetal inflammatory response. Glial cells from the forebrains of 0.65-gestation ovine fetuses (comparable to 24- to 26-week human fetal brain development) were mechanically and enzymatically isolated and plated at a final density of 250,000 cells per well. When reaching confluence at 5 days after plating, the cultures contained astrocytes, microglial cells, as well as progenitor, precursor and immature OLs. Glial cell morphology and phenotypic immunoreactivity were characteristic of and consistent with previous observations of separately cultured cell types. To determine the effects of infection or inflammation in our in vitro model, we then treated mixed glial cultures with tumour necrosis factor-α (TNF-α; 50 or 100 ng/ml) or lipopolysaccharide (LPS; 1 µg/ml) for a period of 48 h. Cytokine levels were measured by ELISA and cell numbers for specific glial cell types were determined along with OL proliferation and apoptosis by Ki67 and caspase-3 immunocytochemistry, respectively. Our results showed that exposure to TNF-α or LPS resulted in a characteristic inflammatory response entailed by up-regulation of pro-inflammatory cytokines, a lack of astrogliosis and a marked reduction in OLs attributable to increased apoptosis. In LPS-treated cultures, there was a marked increase in the pro-inflammatory cytokine TNF-α at both 24 and 48 h. In conclusion, this is the first report of the immunocytochemical description and characterization of fetal ovine-derived mixed glial cell primary cultures. This in vitro model provides a novel and efficient system to explore the mechanisms of infection/inflammation-mediated WMI at the cellular level and for screening candidate therapeutic strategies.

Valproic acid induces microglial dysfunction, not apoptosis, in human glial cultures.

Valproic acid (VPA) is widely used for the treatment of mood disorders and epilepsy, but its mechanism of action is unclear. In vivo and in vitro studies using rodent models have demonstrated that VPA has both neuroprotective and neurotrophic effects. These beneficial effects are, in part, through modulation of glial cell function. Recently, we and others have shown that VPA selectively induces caspase-3 mediated apoptosis in rodent microglial cells. However, the effect of VPA on human microglia has not been tested. In this study, using microglia derived from adult human brains, we demonstrate that VPA does not induce microglial apoptosis as determined by the absence of caspase-3 cleavage. However, VPA does partially decrease the expression of the microglial markers PU.1 and CD45, as well as dramatically reducing microglial phagocytosis. Due to the many roles of microglia in the brain, these VPA-induced alterations in microglial phenotype could potentially have major effects on physiological and pathological actions of these cells.

Up-regulation of the isoenzymes MAO-A and MAO-B in the human basal ganglia and pons in Huntington's disease revealed by quantitative enzyme radioautography.

Huntington's disease (HD) is a rare genetic disease associated with the degeneration of GABAergic striatal projection neurons in the basal ganglia leading to movement disorders with behavioral symptoms for which there is presently no therapy. Abnormally high levels of monoamine oxidase (MAO) activity, which are potentially linked to cytotoxic free radical formation, are known to occur during aging and in neurodegenerative disorders (MAO-B is markedly increased in plaque-associated astrocytes in Alzheimer's disease). We therefore measured, with anatomical resolution, MAO-A and -B activities in 5 cases of HD (severity grades 1-3) and age-matched controls by quantitative enzyme radioautography using radiolabeled enzyme inhibitors (3)H-Ro 41-1049 and (3)H-lazabemide, respectively, as high-affinity ligands in vitro. MAO-A was increased significantly (ca. 50%; p<0.01) in the putamen and substantia nigra pars compacta of the basal ganglia and in the pons. Higher increases in MAO-B (75%-200%; p<0.01) occurred in the putamen, ventral striatum, globus pallidus externus and internus of the basal ganglia and in the insular cortex. The increased enzyme levels (especially of MAO-B) seemed to correlate with the grade of disease severity. We conclude that MAO increases in those regions of HD brains which are known to undergo neurodegeneration accompanied by glioses. Whether or not this increased enzyme activity is a cause or effect of the resulting loss of the GABAergic projection neurons in HD is yet to be clarified. Moreover, it remains to be seen if selective enzyme inhibitors have therapeutic utility in the treatment of HD by reducing oxidative stress locally.

Adult human brain cell culture for neuroscience research.

Studies of the brain have progressed enormously through the use of in vivo and in vitro non-human models. However, it is unlikely such studies alone will unravel the complexities of the human brain and so far no neuroprotective treatment developed in animals has worked in humans. In this review we discuss the use of adult human brain cell culture methods in brain research to unravel the biology of the normal and diseased human brain. The advantages of using adult human brain cells as tools to study human brain function from both historical and future perspectives are discussed. In particular, studies using dissociated cultures of adult human microglia, astrocytes, oligodendrocytes and neurons are described and the applications of these types of study are evaluated. Alternative sources of human brain cells such as adult neural stem cells, induced pluripotent stem cells and slice cultures of adult human brain tissue are also reviewed. These adult human brain cell culture methods could benefit basic research and more importantly, facilitate the translation of basic neuroscience research to the clinic for the treatment of brain disorders.

Cellular composition of human glial cultures from adult biopsy brain tissue.

Microglia and astrocytes play vital roles in normal human brain function and in neurological disorders. To study their physiological and pathological roles it is desirable to establish in vitro systems that are derived from the adult human brain. Although several groups have successfully cultured cells from the human brain, the composition of these cultures remains controversial. Using morphological criteria, immunocytochemical analysis and a BrdU incorporation assay we demonstrate the presence of poorly proliferative microglia and astrocytes in cultures derived from epilepsy biopsy tissue. In addition, we characterized a third cell type as fibronectin and prolyl 4-hydroxylase immunopositive fibroblast-like cells, which are highly proliferative and become the predominant cell type after successive sub-culturing. Therefore, although cultures from adult human brain tissue provide an excellent resource for studying human glial cells, careful consideration must be given to their cellular composition when performing studies using these methods.

Extracellular signal-regulated kinase involvement in human astrocyte migration.

Glial scar formation occurs after virtually any injury to the brain. The migration of astrocytes into regions of brain injury underlies the formation of the glial scar. The exact role of the glial scar has yet to be elucidated, although it is likely to impair brain recovery. Understanding astrocyte migration is fundamental to understanding the formation of the glial scar. We have used human astrocytes (NT2A cells), derived from human NT2/D1 precursor cells to study astrocyte migration using an in vitro scratch wound model. Time-lapse microscopy and bromodeoxyuridine labeling revealed that the astrocytes migrated rather than proliferated across the scratch. Time course immunocytochemical studies showed that scratching human astrocytes induced the activation (phosphorylation) of ERK 1/2 at 10 min after scratch. The MEK 1/2 inhibitor U0126 inhibited both the ERK 1/2 phosphorylation and the migration of the astrocytes across the wound after scratch. Thus, the migration of human astrocytes after injury is partly initiated by activation of the MEK-ERK signalling pathway.

High throughput quantification of cells with complex morphology in mixed cultures.

Automated image-based and biochemical assays have greatly increased throughput for quantifying cell numbers in in vitro studies. However, it has been more difficult to automate the counting of specific cell types with complex morphologies in mixed cell cultures. We have developed a fully automated, fast, accurate and objective method for the quantification of primary human GFAP-positive astrocytes and CD45-positive microglia from images of mixed cell populations. This method, called the complex cell count (CCC) assay, utilizes a combination of image processing and analysis operations from MetaMorph (Version 6.2.6, Molecular Devices). The CCC assay consists of four main aspects: image processing with a unique combination of morphology filters; digital thresholding; integrated morphometry analysis; and a configuration of object standards. The time needed to analyze each image is 1.82s. Significant correlations have been consistently achieved between the data obtained from CCC analysis and manual cell counts. This assay can quickly and accurately quantify the number of human astrocytes and microglia in mixed cell culture and can be applied to quantifying a range of other cells/objects with complex morphology in neuroscience research.

Microglia induce neural cell death via a proximity-dependent mechanism involving nitric oxide.

Microglial cells play a major role in the pathogenesis of many neurological diseases by exacerbating neuronal and non-neuronal cell death, but the mechanisms involved are unclear. To investigate the microglial-neuronal interactions, we used the murine BV-2 microglial cell line and the human neuronal-like SK-N-SH neuroblastoma cell line in a co-culture system that enabled proximity-dependent interaction and communication, a trans-well system that allowed proximity-independent communication through diffusible molecules only, and a conditioned media system through which no proximity-dependent interactions or cell-to-cell communication is possible. Activation of BV-2 cells with lipopolysaccharide and interferon-gamma (LPS/IFN-gamma) decreased viability of the BV-2 cells alone and in co-cultures with SK-N-SH cells, but not SK-N-SH cells grown alone. In contrast, activation of BV-2 cells in the trans-well and conditioned media system did not have any effect on the viability of SK-N-SH cells, suggesting that microglia must be in close proximity to the neural cells to elicit cytotoxicity. To determine the molecules involved in proximity-dependent cell death, inhibitors of microglial activation were investigated. Only the specific inducible nitric oxide synthase (iNOS) inhibitor S-methylisothiourea, and hypothermia, which is known to suppress microglial iNOS expression, prevented cell death after LPS/IFN-gamma activation. These results suggest that activated microglia release nitric oxide that is, at least partially, responsible for proximity-dependent microglial-mediated neural toxicity.