Astrocytes are specialized glial cells that outnumber neurons by over fivefold. They contiguously tile the entire central nervous system (CNS) and exert many essential complex functions in the healthy CNS. Astrocytes respond to all forms of CNS insults through a process referred to as reactive astrogliosis, which has become a pathological hallmark of CNS structural lesions. Substantial progress has been made recently in determining functions and mechanisms of reactive astrogliosis and in identifying roles of astrocytes in CNS disorders and pathologies. A vast molecular arsenal at the disposal of reactive astrocytes is being defined. Transgenic mouse models are dissecting specific aspects of reactive astrocytosis and glial scar formation in vivo. Astrocyte involvement in specific clinicopathological entities is being defined. It is now clear that reactive astrogliosis is not a simple all-or-none phenomenon but is a finely gradated continuum of changes that occur in context-dependent manners regulated by specific signaling events. These changes range from reversible alterations in gene expression and cell hypertrophy with preservation of cellular domains and tissue structure, to long-lasting scar formation with rearrangement of tissue structure. Increasing evidence points towards the potential of reactive astrogliosis to play either primary or contributing roles in CNS disorders via loss of normal astrocyte functions or gain of abnormal effects. This article reviews (1) astrocyte functions in healthy CNS, (2) mechanisms and functions of reactive astrogliosis and glial scar formation, and (3) ways in which reactive astrocytes may cause or contribute to specific CNS disorders and lesions.

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Astrocytes: biology and pathology

Vulnerable periods during the development of the nervous system are sensitive to environmental insults because they are dependent on the temporal and regional emergence of critical developmental processes (i.e., proliferation, migration, differentiation, synaptogenesis, myelination, and apoptosis). Evidence from numerous sources demonstrates that neural development extends from the embryonic period through adolescence. In general, the sequence of events is comparable among species, although the time scales are considerably different. Developmental exposure of animals or humans to numerous agents (e.g., X-ray irradiation, methylazoxymethanol, ethanol, lead, methyl mercury, or chlorpyrifos) demonstrates that interference with one or more of these developmental processes can lead to developmental neurotoxicity. Different behavioral domains (e.g., sensory, motor, and various cognitive functions) are subserved by different brain areas. Although there are important differences between the rodent and human brain, analogous structures can be identified. Moreover, the ontogeny of specific behaviors can be used to draw inferences regarding the maturation of specific brain structures or neural circuits in rodents and primates, including humans. Furthermore, various clinical disorders in humans (e.g., schizophrenia, dyslexia, epilepsy, and autism) may also be the result of interference with normal ontogeny of developmental processes in the nervous system. Of critical concern is the possibility that developmental exposure to neurotoxicants may result in an acceleration of age-related decline in function. This concern is compounded by the fact that developmental neurotoxicity that results in small effects can have a profound societal impact when amortized across the entire population and across the life span of humans.

Critical periods of vulnerability for the developing nervous system: evidence from humans and animal models.

The glial scar and central nervous system repair

Reactive astrocytes: cellular and molecular cues to biological function

Despite a hundred years' research, the neuropathology of schizophrenia remains obscure. However, neither can the null hypothesis be sustained--that it is a 'functional' psychosis, a disorder with no structural basis. A number of abnormalities have been identified and confirmed by meta-analysis, including ventricular enlargement and decreased cerebral (cortical and hippocampal) volume. These are characteristic of schizophrenia as a whole, rather than being restricted to a subtype, and are present in first-episode, unmedicated patients. There is considerable evidence for preferential involvement of the temporal lobe and moderate evidence for an alteration in normal cerebral asymmetries. There are several candidates for the histological and molecular correlates of the macroscopic features. The probable proximal explanation for decreased cortical volume is reduced neuropil and neuronal size, rather than a loss of neurons. These morphometric changes are in turn suggestive of alterations in synaptic, dendritic and axonal organization, a view supported by immunocytochemical and ultrastructural findings. Pathology in subcortical structures is not well established, apart from dorsal thalamic nuclei, which are smaller and contain fewer neurons. Other cytoarchitectural features of schizophrenia which are often discussed, notably entorhinal cortex heterotopias and hippocampal neuronal disarray, remain to be confirmed. The phenotype of the affected neuronal and synaptic populations is uncertain. A case can be made for impairment of hippocampal and corticocortical excitatory pathways, but in general the relationship between neurochemical findings (which centre upon dopamine, 5-hydroxytryptamine, glutamate and GABA systems) and the neuropathology of schizophrenia is unclear. Gliosis is not an intrinsic feature; its absence supports, but does not prove, the prevailing hypothesis that schizophrenia is a disorder of prenatal neurodevelopment. The cognitive impairment which frequently accompanies schizophrenia is not due to Alzheimer's disease or any other recognized neurodegenerative disorder. Its basis is unknown. Functional imaging data indicate that the pathophysiology of schizophrenia reflects aberrant activity in, and integration of, the components of distributed circuits involving the prefrontal cortex, hippocampus and certain subcortical structures. It is hypothesized that the neuropathological features represent the anatomical substrate of these functional abnormalities in neural connectivity. Investigation of this proposal is a goal of current neuropathological studies, which must also seek (i) to establish which of the recent histological findings are robust and cardinal, and (ii) to define the relationship of the pathological phenotype with the clinical syndrome, its neurochemistry and its pathogenesis.

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The neuropathology of schizophrenia. A critical review of the data and their interpretation.

Recent studies of gliosis in a variety of animal models are reviewed. The models include brain injury, neurotoxic damage, genetic diseases and inflammatory demyelination. These studies show that reactive gliosis is not a stereotypic response, but varies widely in duration, degree of hyperplasia, and time course of expression of GFAP immunostaining, content and mRNA. We conclude that there are different biological mechanisms for induction and maintenance of reactive gliosis, which, depending on the kind of tissue damage, result in different expressions of the gliotic response.

Quantitative aspects of reactive gliosis: A review

Primary astrocytes were cultured from forebrains of 1-day-old rats. Immunofluorescence microscopy showed that approximately 80% of the cells were positive for glial fibrillary acidic protein (GFAP) and greater than 80% were stained with an antiserum to the molecular weight 58,000 fibroblast intermediate filament protein (vimentin). Gel electrophoresis of Triton-insoluble cytoskeleton preparations from these cultures revealed three major bands having molecular weights of 58,000, 51,000, and 42,000, together with some prominent lower-molecular-weight species. The protein of molecular weight 51,000 was not present in preparations from fibroblasts. Each of the three major astrocyte proteins was subjected to limited proteolysis, while two of the proteins were cleaved by cyanogen bromide. The electrophoretic peptide patterns of the 58,000 protein were similar to those of vimentin isolated from NIL-8 fibroblasts, and the patterns of the 51,000 protein were similar to those of GFAP isolated from rat spinal cord. The patterns of the protein of molecular weight 42,000 resembled those of muscle action. Rocket immunoelectrophoresis showed that the 51,000 astrocyte protein reacted with an antiserum to bovine GFAP, but the 58,000 and 42,000 proteins failed to react. We conclude that the major proteins of cytoskeleton preparations from cultured primary astrocytes are vimentin (58,000), GFAP (51,000), and actin (42,000), and that our data show no obvious structural relationship among them.

The Cytoskeleton of Primary Astrocytes in Culture Contains Actin, Glial Fibrillary Acidic Protein, and the Fibroblast-Type Filament Protein, Vimentin

We previously reported that the cytoskeleton of rat astrocytes in primary culture contains vimentin, glial fibrillary acidic protein (GFAP), and actin. These proteins were found in a fraction insoluble in Triton X-100 and thought to be assembled in filamentous structures. We now used primary astrocyte cultures to study the kinetics of synthesis and turnover of these cytoskeletal proteins. The intermediate filament proteins were among the most actively synthesized by astrocytes. High levels of synthesis were detectable by the third day of culture in the early log phase of growth, and the pattern of labeling at day 3 was similar to that at 14 days when the cultures had reached confluency. In short-term incorporation experiments vimentin, GFAP, and actin in the Triton-insoluble fraction were labeled within 5 min after exposure of the cultures to radioactive leucine. We did not detect any saturation of labeling for up to 6 h of incubation. The turnover of filament proteins studied by following the decay of radioactivity from prelabeled vimentin, GFAP, and cytoskeletal actin displayed biphasic decay kinetics for all three proteins. In the initial phase a fast-decaying pool with a half-life of 12–18 h contributed about 40% of the total activity in each protein. A major portion, about 60%, of each protein, however, decayed much more slowly, exhibiting a half-life of about 8 days.

Synthesis and Turnover of Cytoskeletal Proteins in Cultured Astrocytes

The three major proteins of mammalian neurofilaments, of molecular weight 70,000, 160,000, and 210,000, have been resolved by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, and more recently, by ion-exchange chromatography in urea solution. We describe here a method to separate the neurofilament proteins by gel filtration without the use of SDS. A bulk preparation of cytoskeleton from rat spinal cord was first characterized. This preparation was then solubilized in a buffer containing 8 M urea and subjected to gel filtration. Individual neurofilament proteins, in milligram quantities, were harvested following the pooling of appropriate fractions. Gel electrophoresis showed a high degree of homogeneity in each of the three pooled fractions. Dye binding studies demonstrated that the protein of molecular weight 210,000 was relatively underrepresented when stained with Coomassie Blue, while all three neurofilament proteins showed similar dye binding properties with Fast Green. Amino acid analysis indicated that (1) all three neurofilament proteins contained a high content of acidic residues; (2) the molecular weight 210,000 protein contained greater than 8 mol% proline; and (3) no simple oligomeric relationship existed among the neurofilament triplets.

Bulk preparation of CNS cytoskeleton and the separation of individual neurofilament proteins by gel filtration: dye-binding characteristics and amino acid compositions.

Fung-Chow Chiu

Papers

Quantitative aspects of reactive gliosis: A review

The Cytoskeleton of Primary Astrocytes in Culture Contains Actin, Glial Fibrillary Acidic Protein, and the Fibroblast-Type Filament Protein, Vimentin

Synthesis and Turnover of Cytoskeletal Proteins in Cultured Astrocytes

Bulk preparation of CNS cytoskeleton and the separation of individual neurofilament proteins by gel filtration: dye-binding characteristics and amino acid compositions.

Biochemical and immunocytochemical changes in glial fibrillary acidic protein after sab wounds