It is now well established that the glial fibrillary acidic protein (GFAP) is the principal 8-9 nm intermediate filament in mature astrocytes of the central nervous system (CNS). Over a decade ago, the value of GFAP as a prototype antigen in nervous tissue identification and as a standard marker for fundamental and applied research at an interdisciplinary level was recognized (Raine, 135). As a member of the cytoskeletal protein family, GFAP is thought to be important in modulating astrocyte motility and shape by providing structural stability to astrocytic processes. In the CNS of higher vertebrates, following injury, either as a result of trauma, disease, genetic disorders, or chemical insult, astrocytes become reactive and respond in a typical manner, termed astrogliosis. Astrogliosis is characterized by rapid synthesis of GFAP and is demonstrated by increase in protein content or by immunostaining with GFAP antibody. In addition to the major application of GFAP antisera for routine use in astrocyte identification in the CNS, the molecular cloning of the mouse gene in 1985 has opened a new and rich realm for GFAP studies. These include antisense, null mice, and numerous promoter studies. Studies showing that mice lacking GFAP are hypersensitive to cervical spinal cord injury caused by sudden acceleration of the head have provided more direct evidence for a structural role of GFAP. While the structural function of GFAP has become more acceptable, the use of GFAP antibodies and promoters continue to be valuable in studying CNS injury, disease, and development.

Glial Fibrillary Acidic Protein: GFAP-Thirty-One Years (1969–2000)

One of the most remarkable characteristics of astrocytes is their vigorous response to diverse neurologic insults, a feature that is well conserved across a variety of different species. The astroglial response occurs rapidly and can be detected within one hour of a focal mechanical trauma (Mucke et al., 1991). Prominent reactive astrogliosis is seen; in AIDS dementia; a variety of other viral infections; prion associated spongiform encephalopathies; inflammatory demyelinating diseases; acute traumatic brain injury; neurodegenerative diseases such as Alzheimer's disease. The prominence of astroglial reactions in various diseases, the rapidity of the astroglial response and the evolutionary conservation of reactive astrogliosis indicate that reactive astrocytes fulfill important functions of the central nervous system (CNS). Yet, the exact role reactive astrocytes play in the injured CNS has so far remained elusive. This chapter summaries the various experimental models and diseases that exhibit astrogliosis and increase in glial fibrillary acidic protein (GFAP). Recent in vitro studies to inhibit GFAP synthesis are also presented.

GFAP and Astrogliosis

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

The astroglial response to CNS injury is considered in the context of neuron-glial relationships. Although previous models suggested that astroglial cells present in "scars" impede axon regrowth owing to irreversible changes in the glial cell following injury, recent in vivo and in vitro studies indicate that astroglial cells exhibit considerable plasticity, elevating expression of the glial filament protein and altering expression of properties which support axons, including extracellular matrix components and cell surface adhesion systems. Both in vivo and in vitro studies on neuron-glia interactions in different brain regions suggest that glia express region-specific properties, including ion channels, neurotransmitter uptake and receptor systems, and cell surface adhesion systems. Together these findings suggest that a more detailed analysis of glial response to injury in different brain regions will lead to an appreciation of the diversity of the astroglial response to injury, and its regulation by neuron-glia relationships.

Astroglia in CNS injury

Purpose To characterize early structural gliotic reactions in retinal Muller cells, astrocytes, and microglia in experimentally induced diabetes. Methods Rats were rendered diabetic by streptozotocin injection and killed after 2, 4, 12, or 20 weeks. Cell densities were determined in flatmounted retinas or transverse semithin sections. Expression of glial fibrillary acidic protein (GFAP) was localized on frozen sections or flatmounts by immunofluorescence and confocal microscopy, and GFAP content was evaluated by Western blot analysis. Microglial cells were visualized by binding of isolectin B4 or staining with antibodies to phosphotyrosine residues. The integrity of the blood-retinal barrier was assessed by intravenous injection of Evans blue. Results The density of Muller cells and microglia was significantly increased at 4 weeks of diabetes compared with nondiabetic controls. GFAP expression in Muller cells was not detected at 4 weeks but was prominent at 12 weeks. The number of astrocytes was significantly reduced at 4 weeks in the peripapillary and far peripheral retina. Shape changes of microglial cells indicated functional activation. Leakage of the blood-retinal barrier was observed at 2 weeks of hyperglycemia, the earliest time point investigated. Conclusions The leakage of the blood-retinal barrier before glial reactivity suggests that glia are early targets of vascular hyperpermeability. The individual glial cell types react differentially to the diabetic state. Muller cells undergo hyperplasia preceding GFAP expression, and microglial cells are activated, whereas astrocytes regress. This glial behavior may contribute decisively to the onset and development of neuropathy in the diabetic retina.

Karin I. Forno

Papers

Glial fibrillary acidic protein in cryogenic lesions of the rat brain.