GFAPbeta mRNA expression in the normal rat brain and after neuronal injury

GFAP Alternative Splicing and the Relevance for Disease – A Focus on Diffuse Gliomas

Glial fibrillary acidic protein (GFAP) is an intermediate filament protein that is characteristic for astrocytes and neural stem cells, and their malignant analogues in glioma. Since the discovery of the protein 50 years ago, multiple alternative splice variants of the GFAP gene have been discovered, leading to different GFAP isoforms. In this review, we will describe GFAP isoform expression from gene to protein to network, taking the canonical isoforms GFAPα and the main alternative variant GFAPδ as the starting point. We will discuss the relevance of studying GFAP and its isoforms in disease, with a specific focus on diffuse gliomas.

Characterization of a panel of monoclonal antibodies recognizing specific epitopes on GFAP

Alexander disease (AxD) is a neurodegenerative disease caused by heterozygous mutations in the GFAP gene, which encodes the major intermediate filament protein of astrocytes. This disease is characterized by the accumulation of cytoplasmic protein aggregates, known as Rosenthal fibers. Antibodies specific to GFAP could provide invaluable tools to facilitate studies of the normal biology of GFAP and to elucidate the pathologic role of this IF protein in disease. While a large number of antibodies to GFAP are available, few if any of them have defined epitopes. Here we described the characterization of a panel of commonly used anti-GFAP antibodies, which recognized epitopes at regions extending across the rod domain of GFAP. We show that all of the antibodies are useful for immunoblotting and immunostaining, and identify a subset that preferentially recognized human GFAP. Using these antibodies, we demonstrate the presence of biochemically modified forms of GFAP in brains of human AxD patients and mouse AxD models. These data suggest that this panel of anti-GFAP antibodies will be useful for studies of animal and cell-based models of AxD and related diseases in which cytoskeletal defects associated with GFAP modifications occur.

Glial fibrillary acidic protein: from intermediate filament assembly and gliosis to neurobiomarker

Glial fibrillary acidic protein (GFAP) is an intermediate filament (IF) III protein uniquely found in astrocytes in the central nervous system (CNS), non-myelinating Schwann cells in the peripheral nervous system (PNS), and enteric glial cells. GFAP mRNA expression is regulated by several nuclear-receptor hormones, growth factors, and lipopolysaccharides (LPSs). GFAP is also subject to numerous post-translational modifications (PTMs), while GFAP mutations result in protein deposits known as Rosenthal fibers in Alexander disease. GFAP gene activation and protein induction appear to play a critical role in astroglial cell activation (astrogliosis) following CNS injuries and neurodegeneration. Emerging evidence also suggests that, following traumatic brain and spinal cord injuries and stroke, GFAP and its breakdown products are rapidly released into biofluids, making them strong candidate biomarkers for such neurological disorders.

Glial fibrillary acidic protein isoform expression in plaque related astrogliosis in Alzheimer's disease

In Alzheimer's disease (AD), amyloid plaques are surrounded by reactive astrocytes with an increased expression of intermediate filaments including glial fibrillary acidic protein (GFAP). Different GFAP isoforms have been identified that are differentially expressed by specific subpopulations of astrocytes and that impose different properties to the intermediate filament network. We studied transcript levels and protein expression patterns of all known GFAP isoforms in human hippocampal AD tissue at different stages of the disease. Ten different transcripts for GFAP isoforms were detected at different abundancies. Transcript levels of most isoforms increased with AD progression. GFAPδ-immunopositive astrocytes were observed in subgranular zone, hilus, and stratum-lacunosum-moleculare. GFAPδ-positive cells also stained for GFAPα. In AD donors, astrocytes near plaques displayed increased staining of both GFAPα and GFAPδ. The reading-frame-shifted isoform, GFAP(+1), staining was confined to a subset of astrocytes with long processes, and their number increased in the course of AD. In conclusion, the various GFAP isoforms show differential transcript levels and are upregulated in a concerted manner in AD. The GFAP(+1) isoform defines a unique subset of astrocytes, with numbers increasing with AD progression. These data indicate the need for future exploration of underlying mechanisms concerning the functions of GFAPδ and GFAP(+1) isoforms in astrocytes and their possible role in AD pathology.

GFAPδ expression in glia of the developmental and adolescent mouse brain

Glial fibrillary acidic protein (GFAP) is the major intermediate filament (IF) protein in astrocytes. In the human brain, GFAP isoforms have unique expression patterns, which indicate that they play distinct functional roles. One isoform, GFAPδ, is expressed by proliferative radial glia in the developing human brain. In the adult human, GFAPδ is a marker for neural stem cells. However, it is unknown whether GFAPδ marks the same population of radial glia and astrocytes in the developing mouse brain as it does in the developing human brain. This study characterizes the expression pattern of GFAPδ throughout mouse embryogenesis and into adolescence. Gfapδ transcripts are expressed from E12, but immunohistochemistry shows GFAPδ staining only from E18. This finding suggests a translational uncoupling. GFAPδ expression increases from E18 to P5 and then decreases until its expression plateaus around P25. During development, GFAPδ is expressed by radial glia, as denoted by the co-expression of markers like vimentin and nestin. GFAPδ is also expressed in other astrocytic populations during development. A similar pattern is observed in the adolescent mouse, where GFAPδ marks both neural stem cells and mature astrocytes. Interestingly, the Gfapδ/Gfapα transcript ratio remains stable throughout development as well as in primary astrocyte and neurosphere cultures. These data suggest that all astroglia cells in the developing and adolescent mouse brain express GFAPδ, regardless of their neurogenic capabilities. GFAPδ may be an integral component of all mouse astrocytes, but it is not a specific neural stem cell marker in mice as it is in humans.

GFAP isoforms in adult mouse brain with a focus on neurogenic astrocytes and reactive astrogliosis in mouse models of Alzheimer disease

Glial fibrillary acidic protein (GFAP) is the main astrocytic intermediate filament (IF). GFAP splice isoforms show differential expression patterns in the human brain. GFAPδ is preferentially expressed by neurogenic astrocytes in the subventricular zone (SVZ), whereas GFAP(+1) is found in a subset of astrocytes throughout the brain. In addition, the expression of these isoforms in human brain material of epilepsy, Alzheimer and glioma patients has been reported. Here, for the first time, we present a comprehensive study of GFAP isoform expression in both wild-type and Alzheimer Disease (AD) mouse models. In cortex, cerebellum, and striatum of wild-type mice, transcripts for Gfap-α, Gfap-β, Gfap-γ, Gfap-δ, Gfap-κ, and a newly identified isoform Gfap-ζ, were detected. Their relative expression levels were similar in all regions studied. GFAPα showed a widespread expression whilst GFAPδ distribution was prominent in the SVZ, rostral migratory stream (RMS), neurogenic astrocytes of the subgranular zone (SGZ), and subpial astrocytes. In contrast to the human SVZ, we could not establish an unambiguous GFAPδ localization in proliferating cells of the mouse SVZ. In APPswePS1dE9 and 3xTgAD mice, plaque-associated reactive astrocytes had increased transcript levels of all detectable GFAP isoforms and low levels of a new GFAP isoform, Gfap-ΔEx7. Reactive astrocytes in AD mice showed enhanced GFAPα and GFAPδ immunolabeling, less frequently increased vimentin and nestin, but no GFAPκ or GFAP(+1) staining. In conclusion, GFAPδ protein is present in SVZ, RMS, and neurogenic astrocytes of the SGZ, but also outside neurogenic niches. Furthermore, differential GFAP isoform expression is not linked with aging or reactive gliosis. This evidence points to the conclusion that differential regulation of GFAP isoforms is not involved in the reorganization of the IF network in reactive gliosis or in neurogenesis in the mouse brain.

GFAP isoforms in adult mouse brain with a focus on neurogenic astrocytes and reactive astrogliosis in mouse models of Alzheimer disease

Glial fibrillary acidic protein (GFAP) is the main astrocytic intermediate filament (IF). GFAP splice isoforms show differential expression patterns in the human brain. GFAPδ is preferentially expressed by neurogenic astrocytes in the subventricular zone (SVZ), whereas GFAP(+1) is found in a subset of astrocytes throughout the brain. In addition, the expression of these isoforms in human brain material of epilepsy, Alzheimer and glioma patients has been reported. Here, for the first time, we present a comprehensive study of GFAP isoform expression in both wild-type and Alzheimer Disease (AD) mouse models. In cortex, cerebellum, and striatum of wild-type mice, transcripts for Gfap-α, Gfap-β, Gfap-γ, Gfap-δ, Gfap-κ, and a newly identified isoform Gfap-ζ, were detected. Their relative expression levels were similar in all regions studied. GFAPα showed a widespread expression whilst GFAPδ distribution was prominent in the SVZ, rostral migratory stream (RMS), neurogenic astrocytes of the subgranular zone (SGZ), and subpial astrocytes. In contrast to the human SVZ, we could not establish an unambiguous GFAPδ localization in proliferating cells of the mouse SVZ. In APPswePS1dE9 and 3xTgAD mice, plaque-associated reactive astrocytes had increased transcript levels of all detectable GFAP isoforms and low levels of a new GFAP isoform, Gfap-ΔEx7. Reactive astrocytes in AD mice showed enhanced GFAPα and GFAPδ immunolabeling, less frequently increased vimentin and nestin, but no GFAPκ or GFAP(+1) staining. In conclusion, GFAPδ protein is present in SVZ, RMS, and neurogenic astrocytes of the SGZ, but also outside neurogenic niches. Furthermore, differential GFAP isoform expression is not linked with aging or reactive gliosis. This evidence points to the conclusion that differential regulation of GFAP isoforms is not involved in the reorganization of the IF network in reactive gliosis or in neurogenesis in the mouse brain.

Identification of a novel vimentin promoter and mRNA isoform

The intermediate filament protein vimentin is involved in a variety of cellular functions both during the normal developmental processes and in human malignancies. We here describe the identification of an alternative vimentin transcript initiating upstream for the canonical vimentin gene promoter and spliced using the vimentin promoter sequence as an intron. Expression analysis showed that the alternative vimentin promoter had the same expression profile as the canonical vimentin gene promoter. The presented data suggest that alternative promoter usage and alternative splicing could be regulatory mechanisms participating in vimentin gene regulation.

The Vps33a gene regulates behavior and cerebellar Purkinje cell number

A mutation in the Vps33a gene causes Hermansky-Pudlak Syndrome (HPS)-like-symptoms in the buff (bf) mouse mutant. The encoded product, Vps33a, is a member of the Sec1 and Class C multi-protein complex that regulates vesicle trafficking to specialized lysosome-related organelles. As Sec1 signaling pathways have been implicated in pre-synaptic function, we examined brain size, cerebellar cell number and the behavioral phenotype of bf mutants. Standardized behavioral tests (SHIRPA protocols) demonstrated significant motor deficits (e.g., grip strength, righting reflex and touch escape) in bf mutants, worsening with age. Histological examination of brain revealed significant Purkinje cell loss that was confirmed with staining for calbindin, a calcium binding protein enriched in Purkinje cells. This pathologic finding was progressive, as older bf mutants (13-14 months) showed a greater attrition of neurons, with their cerebella appearing to be particularly reduced (approximately 30%) in size relative to those of age-matched-control cohorts. These studies suggest that loss of Purkinje neurons is the most obvious neurological atrophy in the bf mutant, a structural change that generates motor coordination deficits and impaired postural phenotypes. It is conceivable therefore that death of cerebellar cells may also be a clinical feature of HPS patients, a pathological event which has not been reported in the literature. In general, the bf mutant may be a potentially new and useful model for understanding Purkinje cell development and function.

Astrocyte growth effects of vascular endothelial growth factor (VEGF) application to perinatal neocortical explants: receptor mediation and signal transduction pathways

The non-angiogenic role of vascular endothelial growth factor (VEGF), and its receptors flt-1 and flk-1, together with downstream signaling pathways were examined in fetal and postnatal rat cerebral cortical organotypic explants. VEGF application in both paradigms caused a significant increase in astroglial proliferation and a dose-dependent increase in GFAP and nestin immunoreactivity. The VEGF receptor flt-1 was observed on most, though not all astrocytes, while flk-1 receptor immunoexpression was absent. Treatment with antisense oligonucleotides (AS-ODNs) to flt-1 resulted in a dramatic decrease in GFAP and nestin immunoreactivity, which further confirmed the role of flt-1 in mediating VEGF's gliotrophic effects, while AS-ODNs to flk-1 had no effect. VEGF-induced gliotrophic effects were found to be mediated by the MAPK/ERK and PI-3 kinase signaling pathways, since the both the MEK1 inhibitor, PD98059 and the PI-3 kinase inhibitor, Wortmannin abolished VEGF-induced astrocytic GFAP(+) expression. Although high dose VEGF application resulted in strong upregulation of both GFAP and nestin immunoreactivity in astrocytes, overlap of the two proteins was not observed in all cells, suggesting that some of the nestin(+) cells might be neural progenitors. Exposure to VEGF resulted in upregulation of both VEGF and bFGF mRNA at the one-day time point, and bFGF protein by 3 days; VEGF activated astrocytes expressed bFGF to a much greater degree than those in untreated explants. The increased expression of bFGF induced by VEGF, may serve in the proliferation of multipotential neural stem/progenitor cells in vitro. VEGF, an established angiogenic factor, appears to play a significant role in the growth and differentiation of astrocytes in the CNS.

Non-invasive imaging of GFAP expression after neuronal damage in mice

Up-regulation of glial fibrillary acidic protein (GFAP) expression is often used as a surrogate marker of neuronal damage. We have created a transgenic mouse line that carries the luciferase gene under the transcriptional control of the mouse GFAP promoter. Biophotonic imaging was used to non-invasively detect the increase in GFAP expression after kainic acid induced neuronal cell death. We demonstrate that after kainic acid treatment, strong biophotonic signals were detected from the brain area. This correlated with both endogenous GFAP and luciferase RNA levels as well as with hippocampal cell death observed histologically. The transgenic mouse line will provide a powerful tool to dynamically monitor neuronal cell death in the living animal and will aid in the discovery and development of drugs to treat damage due to stroke and other neurodegenerative diseases.

Laser microdissection reveals regional and cellular differences in GFAP mRNA upregulation following brain injury, axonal denervation, and amyloid plaque deposition

Astrocytes are one of the major cell types responding to central nervous system injury. Upregulation of the astrocytic intermediate filament molecule glial fibrillary acidic protein (GFAP) is a key event associated with this reaction. To study the response of astrocytes to different types of brain lesions, GFAP mRNA expression was analyzed by quantitative reverse transcription-polymerase chain reaction (qRT-PCR) in mouse brain following injury, axonal denervation (entorhinal cortex lesion), and amyloid plaque deposition (APP23 transgenic mice). Analysis of tissue areas surrounding a lesion revealed a 21-fold increase of GFAP mRNA in tissue surrounding an injury site, a 6-fold increase in denervated tissue areas, and a 5-fold increase in plaque containing tissue. To this GFAP mRNA increase, astrocytic proliferation and migration as well as an increase of cellular GFAP mRNA expression within astrocytes could have contributed. To determine the degree of GFAP mRNA upregulation in individual astrocytes, an immunofluorescence protocol was developed to harvest astrocytes selectively by laser microdissection and preserve intact RNA. qRT-PCR analysis of GFAP mRNA in microdissected astrocytes revealed an 82-fold increase in astrocytes surrounding an injury site, a 30-fold increase in astrocytes located in a denervation zone, and an 18-fold increase in astrocytes surrounding an amyloid plaque. These data demonstrate that GFAP mRNA is strongly upregulated within individual reactive astrocytes in response to a lesion. Because astrocytic GFAP mRNA upregulation differs among the three lesioning paradigms, we conclude that the lesion type is an important determinant of postlesional astrocytic reactivity.

Sarin causes early differential alteration and persistent overexpression in mRNAs coding for glial fibrillary acidic protein (GFAP) and vimentin genes in the central nervous system of rats

Neurotoxic effects of single dose of 0.5 x LD50 sarin (O-isopropylmethylphosphonoflouridate) on central nervous system (CNS) of male Sprague-Dawley rats were studied. We investigated the mRNA expression of the astroglial marker genes glial fibrillary acidic protein (GFAP) and vimentin to evaluate the fate of astroglial and neuronal cells, because reactive gliosis is very often used to assess the extent of CNS damage. Rats were treated with 50 microg/kg/ml of sarin and terminated at the time-points 1 and 2 hours and 1, 3, and 7 days post-treatment. Control rats were treated with normal saline. Total RNA was extracted and Northern blots were hybridized with cDNA probes for GFAP and vimentin, as well as 28S RNA (control). The data obtained indicate that a single dose of sarin (0.5 x LD50) showed induction in the transcript levels of GFAP and vimentin in the cortex, cerebellum, brainstem and midbrain, and spinal cord. The induction showed distinct spatial-temporal differences for each tissue studied. Both GFAP and vimentin were induced at 1 hour in all the tissues studied except brainstem, where moderate and high levels of GFAP induction were noted at 1 and 3 days. Overexpressed transcript levels of GFAP and vimentin remained high in more responsive tissues such as the brainstem and midbrain. Other tissues, such as the cortex, spinal cord, and cerebellum showed a more downward trend for either GFAP or vimentin, or both, transcript levels at 7 days. It is noteworthy that both cortex (318 +/- 12%) and spinal cord (368 +/- 12%) showed relatively higher induction of GFAP, whereas cortex alone showed the highest level of overexpressed vimentin transcript levels (284 +/- 11%). Overall it is also clear that both GFAP and vimentin are needed for the effective recovery involving co-ordinated alternating up- and downregulation of these two key astrocyte genes, depending on tissue specificity. The changes seen in the transcript levels of GFAP and vimentin may be the result of astrocyte dysfunction and loss, accompanied by compensatory proliferation and dedifferentiation of the astroglia. These changes could affect the neuronal cell types, thus altering the neuron-glia homeostasis.