Heterogeneity of astroglia cultured from adult human temporal lobe

Thyroid Hormone and Astrocyte Differentiation

Thyroid hormones (THs) have important contributions to the development of the mammalian brain, targeting its actions on both neurons and glial cells. Astrocytes, which constitute about half of the glial cells, characteristically undergo dramatic changes in their morphology during development and such changes become necessary for the proper development of the brain. Interestingly, a large number of studies have suggested that THs play a profound role in such morphological maturation of the astrocytes. This review discusses the present knowledge on the mechanisms by which THs elicit progressive differentiation and maturation of the astrocytes. As a prelude, information on astrocyte morphology during development and its regulations, the role of THs in the various functions of astrocyte shall be dealt with for a thorough understanding of the subject of this review.

Innate immune responses to HIV infection in the central nervous system

Human immunodeficiency virus (HIV) invades the brain early during infection and generates a chronic inflammatory microenvironment that can eventually result in neurological disease, even in the absence of significant viral replication. Thus, HIV-1 infection of the brain has been characterized both as a neuroimmunological and neurodegenerative disorder. While the brain and central nervous system (CNS) have historically been regarded as immune privileged or immunologically quiescent, newer concepts of CNS immunity suggest an important if not defining role for innate immune responses generated by glial cells. Innate immunity may be the first line of defense against HIV infection of the brain and CNS, with multiple cellular elements providing responses that can be anti-viral and neuroprotective, but also potentially neurotoxic, impairing neurogenesis and promoting neuronal apoptosis. To investigate the effects of HIV exposure on neurogenesis and neuronal survival, we have studied the responses of human neuroepithelial progenitor (NEP) cells, which undergo directed differentiation into astrocytes and neurons in vitro. We identified a group of genes that were differentially expressed in NEP-derived cells during virus exposure. This included genes that are strongly related to interferon-induced responses and antigen presentation. Moreover, we observed that the host factor apolipoprotein E influences the innate immune response expressed by these cells, with a more robust response in the apolipoprotein E3/E3 genotype cultures compared to the apolipoprotein E3/E4 counterparts. Thus, neuroepithelial progenitors and their differentiated progeny recognize HIV and respond to it by mounting an innate immune response with a vigor that is influenced by the host factor apolipoprotein E.

Apolipoprotein E-dependent differences in innate immune responses of maturing human neuroepithelial progenitor cells exposed to HIV-1

HIV enters the brain early during infection and induces a chronic inflammatory state that can result in neurological abnormalities in a subset of infected individuals. To investigate the effects of HIV exposure on neurogenesis and neuronal survival in the brain, we have used a model system consisting of human neuroepithelial progenitor (NEP) cells that undergo directed differentiation into astrocytes and neurons in vitro. Changes in gene expression in NEP cultures as a result of HIV exposure were investigated using gene expression microarrays with the Illumina HT-12 V4_0_R1 platform array. Through this approach, we identified a group of genes specifically upregulated by exposure to virus that are strongly related to interferon induced responses and antigen presentation. When the data were stratified by their apolipoprotein genotype, this innate immune response was more robust in the apolipoprotein E3/E3 genotype cultures than in the apolipoprotein E3/E4 counterparts. Biological processes as defined by the gene ontology (GO) program were also differently affected upon virus exposure in cultures of the two genotypes, particularly those related to antigen presentation and the actions of interferons. Differences occurred in both in numbers of genes affected and their significance in the GO processes in which they participate, with apoE3/E3 > apoE3/E4. These data suggest that maturing NEP cultures recognize HIV and respond to it by mounting an innate immune response with a vigor that is influenced by the apolipoprotein E genotype of the cells.

Astroglial heterogeneity closely reflects the neuronal-defined anatomy of the adult murine CNS

Astroglia comprise an extremely morphologically diverse cell type that have crucial roles in neural development and function. Nonetheless, distinct regions of the CNS have traditionally been defined by the phenotypic characteristics and connectivity of neuros. In a complementary fashion, we present evidence that discrete regions of the adult CNS can be delineated based solely on the morphology, density and proliferation rates of astroglia. We used transgenic hGFAP-GFP mice in which robust expression of GFP in adult astroglia enables detailed morphological characterization of this diversely heterogeneous cell population with 3D confocal microscopy. By using three complementary methods for labeling adult astroglia (hGFAP-GFP expression, and GFAP and S100beta immunostaining), we find that there is a remarkably diverse, regionally stereotypical array of astroglial morphology throughout the CNS, and that discrete anatomical regions can be defined solely on the morphology of astroglia within that region. Second, we find that the density of astroglia varies dramatically across the CNS, and that astroglial density effectively delineates even the sub-regions of complex structures, such as the thalamus. We also find that regional astroglial density varies depending on how astroglia are labeled. To quantify and illustrate these broad differences in astroglial density, we generated an anatomical density atlas of the CNS. Third, the proliferation rate, or mitotic index, of astroglia in the adult CNS also effectively defines anatomical regions. These differences are present regardless of the astroglial-labeling method used. To supplement our atlas of astroglial density we generated an atlas of proliferation density for the adult CNS. Together, these studies demonstrate that the morphology, density and proliferation rate of astroglia can independently define the discrete cytoarchitecture of the adult mammalian CNS, and support the concept that regional astroglial heterogeneity reflects important molecular and functional differences between distinct classes of astroglia, much like the long-accepted heterogeneity of neuronal populations.

Gene expression profiles of HIV-1-infected glia and brain: toward better understanding of the role of astrocytes in HIV-1-associated neurocognitive disorders

Astrocytes are the major cellular component of the central nervous system (CNS), and they play multiple roles in brain development, normal brain function, and CNS responses to pathogens and injury. The functional versatility of astrocytes is linked to their ability to respond to a wide array of biological stimuli through finely orchestrated changes in cellular gene expression. Dysregulation of gene expression programs, generally by chronic exposure to pathogenic stimuli, may lead to dysfunction of astrocytes and contribute to neuropathogenesis. Here, we review studies that employ functional genomics to characterize the effects of HIV-1 and viral pathogenic proteins on cellular gene expression in astrocytes in vitro. We also present the first microarray analysis of primary mouse astrocytes exposed to HIV-1 in culture. In spite of different experimental conditions and microarray platforms used, comparison of the astrocyte array data sets reveals several common gene-regulatory changes that may underlie responses of these cells to HIV-1 and its proteins. We also compared the transcriptional profiles of astrocytes with those obtained in analyses of brain tissues of patients with HIV-1 dementia and macaques infected with simian immunodeficiency virus (SIV). Notably, many of the gene characteristics of responses to HIV-1 in cultured astrocytes were also altered in HIV-1 or SIV-infected brains. Functional genomics, in conjunction with other approaches, may help clarify the role of astrocytes in HIV-1 neuropathogenesis.

TGF-beta1 reduces the heterogeneity of astrocytic cyclooxygenase-2 and nitric oxide synthase-2 gene expression in a stimulus-independent manner

Transforming growth factor-beta1 (TGF-beta1) is upregulated by inflammatory mediators in several neurological diseases/disorders where it either participates in the pathology or provides protection. Often, the biological outcome of TGF-beta1 is dependent upon changes in gene expression. Recently, we demonstrated that TGF-beta1 enhances astrocytic nitric oxide production induced by lipopolysaccharide (LPS) plus interferon-gamma (IFNgamma) by increasing the number of astrocytes in a population that express NOS-2. The purpose of this study was twofold: (1) to determine whether this effect occurs more generally by assessing the effect of TGF-beta1 on another pro-inflammatory gene, cyclooxygenase-2 (COX-2); and (2) to assess stimulus specificity. We found that TGF-beta1 augmented LPS plus IFNgamma-induced COX-2 mRNA and protein expression, by nearly tripling the number of astrocytes that express COX-2. The effect was not stimulus-specific as TGF-beta1 enhanced the number of astrocytes that expressed both COX-2 and NOS-2 protein when either IL-1beta or TNFalpha was used in lieu of LPS. Collectively, these results suggest that TGF-beta1 augments overall protein expression levels of select pro-inflammatory genes in astrocytes in a promiscuous manner by reducing the magnitude of noise in the cellular population.

Human immunodeficiency virus type 1 efficiently binds to human fetal astrocytes and induces neuroinflammatory responses independent of infection

BACKGROUND

HIV-1 infects human astrocytes in vitro and in vivo but the frequency of infected cells is low and its biological significance is unknown. In studies in vitro, recombinant gp120 alone can induce profound effects on astrocyte biology, suggesting that HIV-1 interaction with astrocytes and its functional consequences extend beyond the limited levels of infection in these cells. Here we determined the relative efficiencies of HIV-1 binding and infection in human fetal astrocytes (HFA), mainly at the single cell level, using HIV-1 tagged with green fluorescence protein (GFP)-Vpr fusion proteins, termed HIV-GFP, to detect virus binding and HIV-1 expressing Rev and NefGFP fusion proteins to detect productive infection.

RESULTS

Essentially all HFA in a population bound HIV-GFP specifically and independently of CCR5 and CXCR4. The dynamics of this binding at 37 degrees C resembled binding of an HIV fusion mutant to CD4-positive cells, indicating that most of HIV-GFP arrested infection of HFA at the stage of virus-cell fusion. Despite extensive binding, only about 1% of HFA were detectably infected by HIV-RevGFP or HIV-NefGFP, but this proportion increased to the majority of HFA when the viruses were pseudotyped with vesicular stomatitis virus envelope glycoprotein G, confirming that HFA impose a restriction upon HIV-1 entry. Exposure of HFA to HIV-1 through its native proteins rapidly induced synthesis of interleukin-6 and interleukin-8 with increased mRNA detected within 3 h and increased protein detected within 18 h of exposure.

CONCLUSION

Our results indicate that HIV-1 binding to human astrocytes, although extensive, is not generally followed by virus entry and replication. Astrocytes respond to HIV-1 binding by rapidly increased cytokine production suggesting a role of this virus-brain cell interaction in HIV-1 neuropathogenesis.

Human glial cell culture models of inflammation in the central nervous system

Research into human central nervous system (CNS) disorders has traditionally focused on interconnecting neurons, thought to be the most important functional elements in the CNS. Consequently, animal models have developed as the central paradigm in CNS drug development. However, evidence is accumulating that suggests glial cells play a much more important role in health and disease in the CNS than has been previously acknowledged. Brain development, neurotransmission, inflammatory and neuroprotective pathways and blood-brain barrier functions rely on glial cells. It is also the case that human glial cell cultures adequately mimic in vivo glial cell behaviour, providing a novel and valuable tool for CNS drug discovery and development.

Microarray analysis of changes in cellular gene expression induced by productive infection of primary human astrocytes: implications for HAD

The role of astrocytes in HIV-1 associated dementia (HAD) is not well understood. HIV-1 binds efficiently to astrocytes but infects only a small fraction of the cells in vitro and in vivo. To gain insight into the biology of HIV-1-expressing astrocytes, we productively infected human fetal astrocytes with pseudotyped HIV-1 and employed Affymetrix oligonucleotide microarrays to determine global changes in cellular gene expression at the peak of virus production. With a twofold change as a cutoff, HIV-1 increased transcription of 266 genes in astrocytes and suppressed expression of 468. The functions of highly expressed genes included interferon-mediated antiviral responses (OAS1, IFIT1), intercellular contacts (SH3, glia-derived nexin), cell homing/adhesion (matrix metalloproteinases), and cell-cell signaling (neuropilin 1 and 2). Surprisingly, genes involved in innate immune responses of astrocytes were largely unaffected. The single most significant effect of HIV-1, however, was down-modulation of at least 55 genes involved in control of cell cycle, DNA replication, and cell proliferation, which were overrepresented in these categories with probability scores of 10(-10)-10(-26). Our data suggest that HIV-1 expression in astrocytes profoundly alters host cell biology, with potential consequences for the physiological function of astrocytes during HIV-1 infection in the brain.

Distinct electrophysiological alterations in dentate gyrus versus CA1 glial cells from epileptic humans with temporal lobe sclerosis

Previous studies have characterized the electrophysiological properties of astrocytes in the CA1 region of hippocampi resected from patients with intractable temporal lobe epilepsy (TLE). However, the properties of hilar astrocytes from such patients have not been studied although astrocytes display regional heterogeneity and a non-uniform response to injury. Thus, we performed patch-clamp recordings of putative astrocytes in hilar and CA1 regions of surgically removed epileptic hippocampi with and without sclerosis (mesial TLE, MTLE patients, and paradoxical TLE, PTLE patients, respectively), and non-epileptic, non-sclerotic hippocampi (tumor patients). Our data show that the current profile of hilar astrocytes undergoes significant changes in MTLE but not in PTLE or tumor hippocampi. In particular, inwardly rectifying K(+) (K(IR)) and outwardly rectifying K(+) currents were reduced, inward Na(+) currents and membrane resistances were increased in putative astrocytes from MLTE cases compared to PTLE and tumor cases. Because the conductance of K(IR) channels in cell-attached patches (approximately 34pS) from MTLE tissue was not altered, a reduction in the number of K(IR) channels likely accounts for the decrease in whole-cell K(IR) conductance. Presumed astrocytes in the CA1 region from each patient group displayed intercellular coupling and a passive current profile; these characteristics were never observed in hilar glial cells. No apparent changes in the current profile of coupled CA1 glial cells could be detected between MTLE, PTLE and tumor tissues. Additionally, CA1 glial cells expressed a high density of 34pS K(IR) channels. These data suggest that K(+) buffering via K(IR) channels may be functionally compromised in hilar astrocytes of epileptic and sclerotic (MTLE) human hippocampi. By contrast, CA1 astrocytes retained their intercellular coupling and K(IR) channel expression necessary for K(+) buffering.

Evidence for astrocyte heterogeneity: a distinct subpopulation of protoplasmic-like glial cells is detected in transgenic mice expressing Lmo1-lacZ

The adult mammalian central nervous system (CNS) contains a large number of different cell types, which arise from the ventricular (VZ) and subventricular zones during embryonic development. In this study, we used a transgenic mouse expressing Lmo1-LacZ from a randomly inserted promoter/reporter gene construct to identify a glial subpopulation. LMO1 is an LIM domain-containing protein, thought to act in protein-protein interactions. We found first that in the adult transgenic CNS, beta-galactosidase (beta-gal) was expressed in a specific subpopulation of protoplasmic-like cells, which did not express detectable levels of glial fibrilary acidic protein unless a lesion was produced. Secondly, during development, beta-gal(+) cells were found arising from discrete regions of the VZ. Taken together, these results identify a subpopulation of protoplasmic glial cells in the adult CNS and suggest that they arise from a restricted VZ region during CNS development.

S100B in brain damage and neurodegeneration

S100B is a calcium-binding peptide produced mainly by astrocytes that exert paracrine and autocrine effects on neurons and glia. Some knowledge has been acquired from in vitro and in vivo animal experiments to understand S100B's roles in cellular energy metabolism, cytoskeleton modification, cell proliferation, and differentiation. Also, insights have been gained regarding the interaction between S100B and the cerebral immune system, and the regulation of S100B activity through serotonergic transmission. Secreted glial S100B exerts trophic or toxic effects depending on its concentration. At nanomolar concentrations, S100B stimulates neurite outgrowth and enhances survival of neurons during development. In contrast, micromolar levels of extracellular S100B in vitro stimulate the expression of proinflammatory cytokines and induce apoptosis. In animal studies, changes in the cerebral concentration of S100B cause behavioral disturbances and cognitive deficits. In humans, increased S100B has been detected with various clinical conditions. Brain trauma and ischemia is associated with increased S100B concentrations, probably due to the destruction of astrocytes. In neurodegenerative, inflammatory and psychiatric diseases, increased S100B levels may be caused by secreted S100B or release from damaged astrocytes. This review summarizes published findings on S100B regarding human brain damage and neurodegeneration. Findings from in vitro and in vivo animal experiments relevant for human neurodegenerative diseases and brain damage are reviewed together with the results of studies on traumatic, ischemic, and inflammatory brain damage as well as neurodegenerative and psychiatric disorders. Methodological problems are discussed and perspectives for future research are outlined.

Protein S100 immunoreactivity in glial cells and neurons of the Japanese quail brain

In mammals, sparse data illustrated the neuronal expression of S100 protein in central and peripheral nervous system. Similar studies have not been performed in other vertebrate species, in particular in birds. We provide here a detailed description of the distribution of the calcium-binding protein S100 in neuronal and glial elements in the central nervous system of an avian species, the Japanese quail (Coturnix japonica) largely used for neuroanatomical and functional studies. The distribution of S100-like immunoreactivity was analyzed by three different antisera: a polyclonal, against S100 protein, and two monoclonals, against the beta-subunit (S100beta) and the alpha-subunit (S100alpha) of this protein. All sera showed glial positive elements, which were more abundant in the brainstem than in the prosencephalon. Moreover, the polyclonal and the monoclonal antibodies against the beta-subunit evidenced a neuronal population with a wide distribution, variable morphology and staining intensity. In the telencephalon and diencephalon a few S100-positive neurons were observed in basal ganglia, nucleus paraventricularis hypothalami, nucleus rotundus and nucleus geniculatus lateralis, pars ventralis. In the mesencephalon and pons a wide S100-immunoreactive neuronal population was detected in several regions, including motor and sensory nuclei of most cranial nerves (i.e. oculomotoris, abducens, trigeminus, cochlearis, trochlearis and vestibularis nuclei). This distribution appears very similar to that previously described in the rat hindbrain by both immunocytochemistry and in situ hybridization, as well as to sparse observations on different vertebrates. Therefore, our results suggest that the distribution pattern of this protein (both in glial and in neuronal elements) is highly conserved throughout the phylogeny.

Astrocyte stellation in saline media lacking bicarbonate: possible relation to intracellular pH and tyrosine phosphorylation

Primary cultures of astrocytes exhibit a polygonal morphology, but on treatment with agents that increase cAMP they change to stellate cells. We found that astrocyte stellation also occurred on replacing the culture medium with saline buffered with HEPES. However, stellation did not occur when the medium was replaced with saline buffered with bicarbonate/CO(2) provided Ca(2+) was present. Since exposure of astrocytes to media lacking bicarbonate results in a decrease in intracellular pH (pH(i)) we sought evidence for an association between pH(i) and morphology. Astrocytic pH(i) was monitored for 60 min after transferring the cells to HEPES or bicarbonate-buffered saline. HEPES-induced stellation was associated with transient acidification which coincided with the morphological changes. Acidification was not observed in cells transferred to bicarbonate-saline. However when cytoplasmic acidification of cells in bicarbonate-saline was induced pharmacologically, rapid stellation occurred. Stellation induced by cAMP is reversed by activation of the RhoA pathway with lysophosphatidic acid (LPA). Here we found that LPA inhibited HEPES-induced stellation, but only with Ca(2+) present. Inhibition of stellation by LPA+Ca(2+) was associated with transient acidification followed by modest alkanization. A close association of tyrosine phosphorylation with stellation and pH(i) was observed. Thus incubation of astrocytes in HEPES-saline with orthovanadate to inhibit dephosphorylation abolished stellation and acidification; conversely incubation of cells in bicarbonate-saline with genistein to inhibit tyrosine kinases caused stellation and major acidification. Acidification may be one of several factors resulting in stellation, but it is not a necessary factor since stellation without acidification was observed in bicarbonate-saline lacking Ca(2+).

Immunocontent and secretion of S100B in astrocyte cultures from different brain regions in relation to morphology

Primary astrocyte cultures prepared from neonatal hippocampus, cerebral cortex and cerebellum were morphologically distinct. Cells from hippocampus and cortex were almost entirely protoplasmic, whereas cerebellar astrocytes had many processes; in the absence of serum these differences were accentuated. We compared the immunocontent and secretion of the mitogenic protein S100B in these cultures. Immunocontent was 2.5 times higher in cerebellar astrocytes than in hippocampal or cortical astrocytes. Cells from all three regions secreted S100B under basal conditions, but the secretion rate was higher in cerebellar astrocytes. Secretion depended on protein synthesis and was increased by incubation with forskolin or lysophosphatidic acid in mechanisms which were additive. The stellate morphology induced by forskolin was reversed by lysophosphatidic acid in hippocampal but not in cerebellar cultures, suggesting that S100B secretion was not associated with a process-bearing phenotype of astrocytes.