Irina Rozovsky

Astrocytes and Microglia Respond to Estrogen with Increased apoE mRNAin Vivoandin Vitro

This study examined the regulation of apolipoprotein E (apoE) by 17beta-estradiol (E2) in brain glia, using rats with regular ovulatory cycles as an in vivo model and cultured astrocytes and mixed glia as in vitro models. Two brain regions were examined which had demonstrated transient synaptic remodeling during the estrous cycle. In the hippocampal CA1 region and the hypothalamic arcuate nucleus, apoE mRNA was elevated at proestrus when plasma E2 was high and synaptic density was increasing. Both astrocytes and microglia contributed to this increase in apoE mRNA. In vitro, E2 treatment had no effect on apoE mRNA levels in monotypic cultures of either astrocytes or microglia. In contrast, mixed glial cultures responded to E2 with increased apoE mRNA and protein, suggesting that heterotypic cellular interactions are important in the brain response to estrogens. In situ hybridization in combination with cell-specific markers showed that E2 increased apoE mRNA levels in both astrocytes and microglia. These results, which are the first evidence of apoE mRNA localization to microglia in vivo and the control of apoE expression in brain cells by estrogens, are discussed in terms of the possible protective role of E2 in Alzheimers disease and prior findings that emphasize the expression of apoE mRNA in astrocytes within the brain.

Glial fibrillary acidic protein: regulation by hormones, cytokines, and growth factors.

Levels of glial fibrillary acidic protein (GFAP), an astrocyte‐specific intermediate filament protein, are altered during development and aging, GFAP also responds dynamically to neurodegenerative lesions. Changes in GFAP expression can occur at both transcriptional and translational levels. Modulators of GFAP expression include steroids, cytokines, and growth factors. GFAP expression also shows brain region‐specific responses to sex steroids and of astrocyte‐neuronal interactions. The 5′‐upstream sequences of rat, mouse, and human are compared for the presence of response elements that are candidates for transcriptional regulation of GFAP. We propose that the regulation of the GFAP gene has evolved a system of controls that allow integrated responses to neuroendocrine and inflammatory modulators.

Complement mRNA in the mammalian brain: Responses to Alzheimer's disease and experimental brain lesioning

This study describes evidence in the adult human and rat brain for mRNAs that encode two complement (C) proteins, C1qB and C4. C proteins are important effectors of humoral immunity and inflammation in peripheral tissues but have not been considered as normally present in brain. Previous immunocytochemical studies showed that C proteins are associated with plaques, tangles, and dystrophic neurites in Alzheimers disease (AD), but their source is unknown. Combined immunocytochemistry and in situ hybridization techniques show C4 mRNA in pyramidal neurons and C1qB mRNA in microglia. Primary rat neuron cultures also show C1qB mRNA. In the cortex from AD brains, there were two- to threefold increases of C1qB mRNA and C4 mRNA, and increased C1qB mRNA prevalence was in part associated with microglia. As a model for AD, we examined entorhinal cortex perforant path transection in the rat brain, which caused rapid increases of C1qB mRNA in the ipsilateral, but not contralateral, hippocampus and entorhinal cortex. The role of brain-derived acute and chronic C induction during AD and experimental lesions can now be considered in relation to functions of C proteins that pertain to cell degeneration and/or cell preservation and synaptic plasticity.

The mosaic of brain glial hyperactivity during normal ageing and its attenuation by food restriction.

Food restriction of adult rodents increases lifespan, with commensurate attenuation of age-related pathological lesions in many organs, as well as attenuation of normal ageing changes that are distinct from gross lesions. Previous work showed that chronic food restriction attenuated age-associated astrocyte and microglial hyperactivity in the hippocampal hilus, as measured by expression of glial fibrillary acidic protein and major histocompatibility complex II antigen (OX6). Here, we examined other markers of astrocyte and microglial activation in gray and white matter regions of ad libitum-fed (Brown Norway x Fischer 344) F1 male rats aged three and 24 months and chronic food-restricted rats aged 24 months. In situ hybridization and immunohistochemical techniques evaluated glial expression of glial fibrillary acidic protein, apolipoprotein E, apolipoprotein J (clusterin), heme oxygenase-1, complement 3 receptor (OX42), OX6 and transforming growth factor-beta1. All markers were elevated in the corpus callosum during ageing and were attenuated by food restriction, but other regions showed marked dissociation of the extent and direction of changes. Astrocytic activation, as measured with glial fibrillary acidic protein expression (coding and intron-containing RNA, immunoreactivity), increased with age in the corpus callosum, basal ganglia and hippocampus. Generally, food restriction attenuated the age-related increase in glial fibrillary acidic protein messenger RNA and immunoreactivity. Food restriction also reduced the age-related increase in apolipoprotein J and E messenger RNA and heme oxygenase-1 immunoreactivity in the basal ganglia and corpus callosum. However, astrocytes in the hilus of the hippocampus showed an age-related decrease in apolipoprotein J and E messenger RNA, which was further intensified by food restriction. The age-associated microglial activation measured by OX6 and OX42 immunoreactivity was reduced by food restriction in most subregions. The localized subsets of glial age changes and effects of food restriction comprise a mosaic of ageing consistent with the regional heterogeneity of ageing changes reported by others. In particular, age has a differential effect on astrocytic and microglial hyperactivity in gray versus white matter areas. The evident mosaic of glial ageing and responses to food restriction suggests that multiple mechanisms are at work during ageing.

Complement C1qB and C4 mRNAs responses to lesioning in rat brain

These data show the presence of mRNAs for two complement components (C) in the adult rat brain and describe their responses to experimental lesions. Cortical deafferentation caused elevations in striatal C1qB and C4 mRNAs that coincided temporally and overlapped anatomically with the course of degeneration of corticostriatal afferent fibers. By in situ hybridization, C1qB mRNA in the lesioned striatum was colocalized to cells immunoreactive for CR3, a complement receptor found on microglia-macrophages. The mRNA for SGP-2, a putative C inhibitor in rat, showed parallel changes. Similarly, in hippocampus and other brain regions, kainic acid lesions increased C1qB mRNA. The data suggest that microglia-macrophages and possibly other cells in rat brain rapidly up-regulate C-mRNAs in response to deafferentation and local neuron injury. These experimental responses provide models to analyze changes in C components during Alzheimers disease and other chronic neurodegenerative conditions.

Transforming growth factor-β1 induces neuronal and astrocyte genes: Tubulin α1, glial fibrillary acidic protein and clusterin

Transforming growth factor-beta 1 was studied as a possible regulator of messenger RNAs in astrocytes and neurons that increase after hippocampal deafferentation by perforant path transection: tubulin alpha 1, clusterin and glial fibrillary acidic protein messenger RNA. Because transforming growth factor-beta 1 messenger RNA is increased after this lesion, we examined which messenger RNA lesion responses could be induced by transforming growth factor-beta 1 alone. Porcine transforming growth factor-beta 1 infused into the lateral ventricle elevated the messenger RNAs for tubulin alpha 1, clusterin and glial fibrillary acidic protein 24 h after infusion in the ipsilateral hippocampus. As assayed by nuclear run-on, the transcription of glial fibrillary acidic protein RNA was increased in the ipsilateral hippocampus after perforant path transection and in primary rat astrocyte cultures by transforming growth factor-beta 1. In contrast, transforming growth factor-beta 1 did not change apolipoprotein-E messenger RNA or transcription, or growth associated protein-43 messenger RNA levels. We conclude that transforming growth factor-beta 1 increases subsets of neuronal and astrocyte messenger RNAs coding for cytoskeletal proteins that are also elevated in response to experimental lesions and Alzheimers disease. This suggests that transforming growth factor-beta 1 might be a local organizing factor of neuronal and astrocyte responses to brain injury.

Aging and glial responses to lipopolysaccharide in vitro: greater induction of IL-1 and IL-6, but smaller induction of neurotoxicity

Glial activation during aging was analyzed in primary glia cultured from brain regions sampled across the life span. An initial study showed that phenotypes of activated astrocytes and microglia from aging rat cerebral cortex persisted in primary cultures (Neurobiol. Aging 19 (1998), 97). We extend these findings by examining effects of age on the activation of glial cultures from adult rat brain in response to lipopolysaccharide (LPS), an inflammatory stimulus. Mixed glia from F344 male rats, aged 3 and 24 months, cultured from cerebral cortex (Cx), hippocampus (Hc), and striatum (St), were assayed for cytokines implicated in Alzheimers disease: IL-1alpha, IL-1beta, IL-6, and TNF-alpha. Regional differences across all age groups included consistently lower expression of these cytokines in glia derived from Cx than Hc and St. Aging increased basal IL-6 mRNA and secretion by >or=3-fold in glia from Cx and Hc. Aging also increased LPS-induced IL-1 and IL-6 in Hc more than in Cx, whereas no significant effects of age were seen in St-derived glial cytokines. TNF-alpha secretion did not differ by donor age (basal or LPS-induced). Nitric oxide production by microglia from Cx of aging brains showed a smaller induction in response to LPS, with proportionately less neurotoxicity. Thus, glial activation during aging shows regional selectivity in cytokine expression, with opposite effects of aging on the increased inducibility of IL-1 and IL-6 vs the decreased production of nitric oxide.

Increased transcription of the astrocyte gene GFAP during middle-age is attenuated by food restriction: implications for the role of oxidative stress.

Glial fibrillary acidic protein (GFAP), an intermediate filament of astrocytes, shows increased expression during aging. Because we found that chronic food restriction retards the increase of GFAP mRNA in aging rats and because food restriction decreases the load of oxidized proteins and lipids in association with increased life span, we investigated the regulation of GFAP during oxidative stress and aging. First, we showed that food restriction decreased the transcription of GFAP in aging rats. This result generalizes effects of food restriction on age changes of transcription; whether transcription decreases during aging as in hepatic genes, or increases during aging as in astrocytic GFAP, food restriction attenuates the age change. Moreover, food restriction decreased microglial activation during aging, which suggested the hypothesis that GFAP expression is sensitive to oxidative stress. Because GFAP transcription in cultured glia is increased by oxidative stress in response to hydrogen peroxide and cysteamine whether or not microglia were present, we conclude that responses of GFAP to oxidative stress in astrocytes do not depend on microglial activation. The results implicate oxidative stress in the increased expression of GFAP during aging, but also in responses to brain injury.

Selective expression of clusterin (SGP-2) and complement C1qB and C4 during responses to neurotoxinsin vivo andin vitro

This study concerns expression of the genes encoding three multifunctional proteins: clusterin and two complement cascade components, C1q and C4. Previous work from this and other laboratories has established that clusterin, Clq and C4 messenger RNAs are elevated during Alzheimers disease, and in response to deafferenting and excitotoxic brain lesion. This study addresses hippocampal clusterin, ClqB and C4 expression in response to neurotoxins that caused selective neuron death. Kainate, which preferentially kills hippocampal CA3 pyramidal neurons but not dentate gyrus granule neurons induced clusterin immunoreactivity in CA1 and CA3 pyramidal neurons and adjacent astrocytes, but not in dentate gyrus granule neurons. In contrast, colchicine, which preferentially kills the dentate gyrus granule neurons, induced clusterin immunoreactivity in the local neuropil as punctate deposits, but not in the surviving or degenerating dentate gyrus granule neurons. Clusterin messenger RNA was increased in astrocytes. ClqB and C4 messenger RNAs increased within 48 h after kainate injections, particularly in the CA3 pyramidal layer, less in the dentate gyrus-CA4, and less in CA1. Clq immunoreactivity was detected in CA1 pyramidal neurons and also as small punctate deposits in the CA1 region at eight and 14 days after kainate. The increase of both clusterin and ClqB messenger RNAs after kainate injections was blocked by barbiturates that prevented seizures and neurodegeneration. In primary hippocampal neuronal cultures treated with glutamate, a subpopulation of cultured neurons that survived glutamate toxicity also had parallel elevations of clusterin and ClqB messenger RNA. In conclusion, cytotoxins that target selective hippocampal neurons increase the expression of both clusterin and ClqB in vivo and in vitro. These results show that elevations of clusterin messenger RNA or protein can be dissociated from each other and from cell death. These increased messenger RNAs were associated with immunoreactive deposits that differed by cell type and intra- versus extracellular locations. These results suggest that the complement system is involved in brain responses to injury.

Glial Fibrillary Acidic Protein Transcription Responses to Transforming Growth Factor‐β1 and Interleukin‐1β Are Mediated by a Nuclear Factor‐1‐Like Site in the Near‐Upstream Promoter

Abstract: Elevated expression of glial fibrillary acidic protein (GFAP) is associated with astrocyte activation during responses to injury in the CNS. Because transforming growth factor‐β1 (TGF‐β1) and interleukin‐1β (IL‐1β) are released during neural responses to injury and because these cytokines also modulate GFAP mRNA levels, it is of interest to define their role in GFAP transcription. The increases of GFAP mRNA in response to TGF‐β1 and decreases in response to IL‐1β were shown to be transcriptionally mediated in rat astrocytes transfected with a luciferase‐reporter construct containing 1.9 kb of 5′‐upstream rat genomic DNA. Constructs containing sequential deletions of the rat GFAP 5′‐upstream promoter identified a short region proximal to the transcription start (‐106 to ‐53 bp) that provides full responses to TGF‐β1 and IL‐1β. This region contains an unusual sequence motif with overlapping nuclear factor‐1 (NF‐1)‐ and nuclear factor‐κB (NF‐κB)‐like binding sites and homology to known TGF‐β response elements. Mutagenesis (3‐bp exchanges) in ‐70 to ‐68 bp blocked the induction of GFAP by TGF‐β1 and the repression by IL‐1β. Gel shift experiments showed that the DNA segment ‐85 to ‐63 bp was bound by a factor(s) in nuclear extracts from astrocytes. The concentrations of these DNA binding factors were increased by treatment of astrocytes with TGF‐β1 and decreased by IL‐1β. Binding of these nuclear factors was blocked by mutation of ‐70 to ‐68 bp. Despite homology to NF‐1 or NF‐κB binding sites in the GFAP promoter at segment ‐79 to ‐67 bp, anti‐NF‐κB or anti‐NF‐1 antibodies did not further retard the gel shift of the nuclear factors/DNA complex. Moreover, astrocytic nuclear proteins do not compete for the specific binding to NF‐1 consensus sequence. Thus, nuclear factors from astrocytes that bind to the ‐85‐ to ‐63‐bp promoter segment might be only distantly related to NF‐1 or NF‐κB. These findings are pertinent to the use of GFAP promoter constructs in transgenic animals, because cis‐acting elements in the GFAP promoter are sensitive to cytokines that may be elaborated in response to expression of transgene products.