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

Background Multiple sclerosis is an inflammatory demyelinating disease of the central nervous system and is the most common cause of neurologic disability in young adults. Despite antiinflammatory or immunosuppressive therapy, most patients have progressive neurologic deterioration that may reflect axonal loss. We conducted pathological studies of brain tissues to define the changes in axons in patients with multiple sclerosis. Methods Brain tissue was obtained at autopsy from 11 patients with multiple sclerosis and 4 subjects without brain disease. Fourteen active multiple-sclerosis lesions, 33 chronic active lesions, and samples of normal-appearing white matter were examined for demyelination, inflammation, and axonal pathologic changes by immunohistochemistry and confocal microscopy. Axonal transection, identified by the presence of terminal axonal ovoids, was detected in all 47 lesions and quantified in 18 lesions. Results Transected axons were a consistent feature of the lesions of multiple sclerosis...

Axonal transection in the lesions of multiple sclerosis.

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.

Publisher Summary This chapter discusses the role of plasminogen activators in various biological processes. In specific, it describes two types of plasminogen activators—namely, the urokinase-type plasminogen activator (u-PA) and the tissue-type plasminogen activator (t-PA), which are essentially different gene products. The amino acid sequences of these activators and nucleotide sequences of the corresponding cDNAs have largely been determined, and the cDNAs have been cloned using recombinant techniques. A variety of enzymatic as well as immunological assay and detection methods have also been developed that allows a precise quantification of the activators, a distinction between u-PA and t-PA, determination of whether an activator is present in its active or zymogen form, analysis of the kinetics of different steps of the cascade reaction, and immunocytochemical identification of u-PA and t-PA in tissue sections. Much of the studies on plasminogen activators and cancer has been guided by the hypothesis that proteolysis of the components of extracellular matrix, initiated by the release of plasminogen activator from the cancer cells, plays a decisive role for the degradation of normal tissue, and thereby for invasive growth and metastases.

Plasminogen activators, tissue degradation, and cancer.

Five chemically distinct classes of intermediate filaments can be identified within higher eukaryotic cells. Each class is characteristic of a particular cell type. These filaments may function to integrate mechanically the various structures of the cytoplasmic space in a way that is tailored to the differentiated state of the cell.

Intermediate filaments as mechanical integrators of cellular space

— A procedure is described for the preparation from rat brain of myelin having the same degree of purity at all ages. Such a procedure is essential for the study of myelin composition during development. Microsomal contamination was successfully eliminated by adjusting the method to maintain a constant amount of brain per unit volume in the initial density gradient step, and by including two osmotic shocks and two low-speed centrifugation steps. Myelin prepared in this way from animals ranging from 15 days to 14months of age had a total ATPase activity of 0.3-2.0 μmol of Pi.h−1.mg−1 dry wt of myelin, representing 0.1-1.2 per cent recovery of the total homogenate activity; a Na+, K+- ATPase activity of 0.1-1.6 μfnol of Pi.h−1.mg−1 dry wt, representing 0.04-1.5 per cent recovery; a nucleic acid content of 0.2-0.7 per cent of myelin dry wt, representing 0.2-2.0 per cent recovery; and a ganglioside NANA content of 0.04-0.07 per cent of myelin dry wt. representing 0.2-4.6 per cent recovery. The myelin prepared from 20-day animals had the highest content of the first three constituents; otherwise the values of the four constituents were relatively constant per unit weight of myelin. The amounts of nucleic acid and ganglioside recovered in the myelin fractions increased with increasing age and myelin yield.

Myelination in rat brain: method of myelin isolation.

The cerebro-hepato-renal syndrome is a rare familial malady with cerebral, renal, and skeletal abnormalities, severe hypotonia, cirrhosis, iron and lipid storage, and death within 6 months. Correlated electron microscopic, histochemical, and biochemical studies demonstrate defects in two oxidative organelles. Peroxisomes cannot be found in hepatocytes and renal proximal tubules. In hepatocytes and cortical astrocytes, mitochondria are distorted in their appearance and glycogen stores are increased. Oxygen consumnption of brain and liver mitochondrial preparations with succinate and with substrates reducing nicotinamide adenine dinucleotide is markedly diminished, but the consumption is normal with ascorbate and tetramethylphenylenediamine, which suggests a defect in electron transport prior to the cytochromes. Histochemical studies of mitochondrial oxidation point to a defect between the succinate dehydrogenase flavoprotein and coenzyme Q, possibly in the region of nonheme iron protein.

https://science.sciencemag.org/content/sci/182/4107/62.full.pdf

Peroxisomal and Mitochondrial Defects in the Cerebro-Hepato-Renal Syndrome

— Myelin was isolated from rat brains during development by a procedure giving fractions of constant purity at all ages. The lipid composition of these fractions and of whole brains of littermates was determined. The amount of myelin recovered per brain was a nearly linear function of the logarithm of age from the youngest (15 days) to the oldest (425 days) animals studied. With the exception of the earliest age point, the isolated myelin accounted for approximately 40 per cent of total brain galactolipid, evidence that a constant fraction (calculated to be 60 per cent) of myelin was recovered at all ages. Although the lipid-protein ratio of the myelin was constant with age, marked changes were seen in the amounts of cerebroside, sulphatide, phosphatidylcholine and desmosterol. The total galactolipid increased from 21 per cent of the total lipid at age 15 days to about 31 per cent at maturity. Phosphatidylcholine decreased from 17 to 11 per cent during the same period. Desmosterol decreased from 2.5 per cent of the total sterol to 0.2-0.3 per cent. All of these changes were complete between 2 and 5 months of age; no other ‘lower phase’ lipids showed significant changes with age. Although qualitatively similar to those reported by others, the changes differed in magnitude, with more stability in the levels of cholesterol and phosphatidalethanolamine with development. A sensitive indicator of the maturation of myelin was the mole ratio galactolipid/phosphatidylcholine, which varied from 1.2 at age 15 days to 2.8 at maturity. The maximum rate of myelination occurred at 20 days of postnatal age when myelin was deposited at the rate of 3.5 mg day−1 brain−1. However, at this age the rat brain had only 15 per cent of its eventual complement of myelin. The rate of accumulation of cerebroside in the whole brain paralleled that of myelin, and was the only lipid to show this relationship. Myelin deposition appeared to be almost solely responsible for the continued increase in brain weight after about 100 days of age.

Myelination in rat brain: changes in myelin composition during brain maturation

The effect of cytokines on astrocytes cultured from mature bovine brain was determined both in a serum-containing medium and in a chemically-defined medium. The results showed that in serum-free medium, human TNF and, to a lesser degree, IL-6 and lymphotoxin, were mitogenic for astrocytes. Increased uptake of [3H]thymidine could be detected within 36 h in vitro and its presence in astrocytes was confirmed by autoradiography. In contrast, neither IL-1 alpha nor IL-1 beta induced astrocyte proliferation in serum-free medium but showed some synergistic effect with serum after 72 h. The proliferative effect of TNF and IL-6 was confirmed by cell counting. None of the cytokines tested was toxic for astrocytes as measured by 51Cr release. No mitogenic effect for oligodendroglia, purified from the same source, was detected. The results support a role for products of activated inflammatory cells in the development of astrocyte proliferation that may contribute to the reactive gliosis found in white matter diseases of the central nervous system such as multiple sclerosis.

Proliferation of astrocytes in vitro in response to cytokines. A primary role for tumor necrosis factor.

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.

William T. Norton

Papers

Myelination in rat brain: method of myelin isolation.

Peroxisomal and Mitochondrial Defects in the Cerebro-Hepato-Renal Syndrome

Myelination in rat brain: changes in myelin composition during brain maturation

Proliferation of astrocytes in vitro in response to cytokines. A primary role for tumor necrosis factor.

Quantitative aspects of reactive gliosis: A review