Francine A. Tansey

Glutamine Synthetase in Oligodendrocytes and Astrocytes: New Biochemical and Immunocytochemical Evidence

Abstract: The results of recent immunocytochemical experiments suggest that glutamine synthetase (GS) in the rat CNS may not be confined to astrocytes. In the present study, GS activity was assayed in oligodendrocytes isolated from bovine brain and in oligodendrocytes, astrocytes, and neurons isolated from rat forebrain, and the results were compared with new immunochemical data. Among the cells isolated from rat brain, astrocytes had the highest specific activities of GS, followed by oligodendrocytes. Oligodendrocytes isolated from white matter of bovine brain had GS specific activities almost fivefold higher than those in white matter homogenates. Immunocytochemical staining also showed the presence of GS in both oligodendrocytes and astrocytes in bovine forebrain, in three white‐matter regions of rat brain, and in Vibratome sections as well as paraffin sections.

A pi form of glutathione-S-transferase is a myelin- and oligodendrocyte-associated enzyme in mouse brain.

Abstract: The pi form of glutathione‐S‐transferase (GST), previously found to be oligodendrocyte associated, has also been found in myelin. In the brains of adult mice, immunocytochemical staining for a pi form of GST was observed in myelinated fibers, as well as oligodendrocytes. In contrast, and as previously found in rats, positive immunostaining for mu forms occurred in astrocytes and not in oligodendrocytes or myelinated fibers. Double immunofluorescence staining strengthened the conclusion that pi was in oligodendrocytes and myelin in mouse brains. The results of enzyme assays performed on brain homogenates and purified myelin indicated that GST specific activities in mouse brain myelin were almost as high (0.8‐fold) as those in mouse brain homogenates. Low, but reproducible, GST activities were also observed in myelin purified from rat brains, in which pi had been demonstrated in oligodendrocytes and mu in astrocytes. The pi form was also stained by the immunoblot technique in myelin purified from mouse brain. It was concluded that pi is a myelin associated, as well as oligodendrocyte associated, enzyme in mouse brain, and possibly also in rat brain. This is the first report of localization of GSTs in mouse brain and of GST in myelin.

Characterization and Localization of Glutathione‐S‐Transferases in Rat Brain and Binding of Hormones, Neurotransmitters, and Drugs

Abstract: Rat brain glutathione‐S‐transferases are rich in Yb type subunits with major RNA transcripts coding for a relatively uncommon Yb3 form. The Yb‐containing isoenzymes of brain cytosol bind glucocorticoids and are covalently labeled with dexamethasone 21‐methanesulfonate. Certain neurotransmitters, hormones, and drugs, such as serotonin, dopamine, glucocorticoids, thyroxine, apomorphine, and benzodiazepine derivatives, are effective inhibitors of brain glutathione transferase activity. Immunocytochemical studies show that Yb forms are localized in ependymal cells, subventricular zone cells, astrocytes, tanycytes, and astrocyte foot processes on blood vessels, but Yb was not detected in oligodendrocytes or neurons. Based on their localization and binding properties, brain glutathione‐S‐transferases have the potential to function in intracellular binding of a variety of compounds and thereby govern their uptake and release in brain, transport to neurons, as well as in their detoxification.

Differential Localization of Glutathione‐S‐Transferase Yp and Yb Subunits in Oligodendrocytes and Astrocytes of Rat Brain

Abstract: Glutathione‐S‐transferase Yb subunits were recently identified in rat brain and localized to astrocytes, ependymal cells lining the ventricles, subventricular zone cells, and tanycytes. Another isoform, Yp (π family), was detected in rat brain by immunoblotting, and its mRNA was detected by Northern hybridizations. Double immunofluorescence localized Yb and Yp in different glial cells. The strongly Yp‐positive cells were identified as oligodendrocytes by virtue of their arrangement in rows in white‐matter tracts, colocalization in strongly carbonic anhydrase‐positive cells, and association with myelinated tracts in the corpus striatum. Ependymal cells in the choroid plexus and ventricular lining were also strongly Yp positive, whereas Yb was not detected in the choroid plexus. The occurrence of Yp at low levels in astrocytes was indicated after immunostaining by a sensitive peroxidase‐antiperoxidase method, which revealed weak staining of those cells in the molecular layer of the cortex. The data suggest that Yb and Yp subunits are primarily localized to astrocytes and oligodendrocytes, respectively, and that both are absent from neurons. The glutathione‐S‐transferase in oligodendrocytes may participate in the removal of toxins from the vicinity of the myelin sheath. The finding of glutathione‐S‐transferases in ependymal cells and astrocytes in the brain also suggests that this enzyme could be a first line of defense against toxic substances.

Carbonic anhydrase immunostaining in astrocytes in the rat cerebral cortex

Abstract: Carbonic anhydrase is known to occur in the choroid plexus, oligodendrocytes, and myelin, and to be virtually absent from neurons, in the mammalian CNS; however, there is significant controversy whether it is also present in astrocytes. When brain sections from adult rats were stained for simultaneous immunofluorescence of carbonic anhydrase and the astrocyte marker glutamine synthetase, both antigens were detected in the same glial cells in the cortical gray matter, whereas the oligodendrocytes and myelinated fibers in and adjacent to the white matter showed immunofluorescence only for carbonic anhydrase. Some glial cells in the gray matter also showed double immunofluorescence for carbonic anhydrase and glial fibrillary acidic protein. These results indicate that there is carbonic anhydrase in some astrocytes in the mammalian CNS.

Quantitation of mast cells in experimental allergic neuritis

The number and extent of degranulation of mast cells in the sciatic nerve have been determined in Lewis rats inoculated with bovine intradural root myelin to induce experimental allergic neuritis (EAN). No significant change in either the number of mast cells or the percentage of degranulated cells was observed before eight days post-inoculation (dpi). On day eight, however, there was a significant decrease in the number of detectable mast cells, and on day nine there was a significant increase in the percentage of degranulated cells. These changes immediately preceded the onset of clinical signs of EAN that occurred ten dpi. The number of detectable mast cells remained low throughout the acute stage of the disease, but by three weeks post-inoculation (wpi) there was a significant increase in the number of mast cells (or basophils) that persisted through 16 wpi. The increase in the percentage of degranulated mast cells also persisted through 16 wpi. These results suggest that mast cells participate in the development of EAN in the Lewis rat.

Protection against experimental allergic neuritis with silica quartz dust

Lewis rats sensitized against peripheral nervous system antigens can be protected against experimental allergic neuritis (EAN) by the intraperitoneal injection of silica quartz dust. Two doses of 200 mg silica given 8 and 11 days post-inoculation (dpi) protected against the development of both clinical and pathological disease. A single dose of 200 mg silica 8 dpi gave significant protection against clinical disease but all animals developed pathological signs. A single injection of 200 mg silica 11 dpi, after the onset of early signs, protected against further progression of disease. The protection was long lasting. Given the known toxic effects of silica for macrophages, these results would support the conclusion that macrophages function during the effector stage in the clinical and pathological expression of EAN in the Lewis rat.

Delayed onset of experimental allergic neuritis in rats treated with reserpine.

In delayed-type hypersensitivity most of the invading cells are not specifically sensitized against the initiating antigen but are augmenting cells called in by inflammatory mediators. It has been suggested that vasoactive amines, such as the monoamine serotonin, released by the action of sensitized T-cells on mast cells, may participate in the perivascular emigration of these cells that do not normally leave the blood. To test this hypothesis in experimental allergic neuritis (EAN), Lewis rats sensitized on day zero were treated with a single dose of the monoamine-depleting drug reserpine (2.5 mg/kg) immediately before the onset of early clinical signs on day nine (reserpine day 9, Rd9); during the onset of early clinical signs on day ten (Rd10); or immediately after the onset of early clinical signs on day 11 (Rd11). The results showed that the onset of early clinical signs was delayed in the Rd9 treated rats until approximately day thirteen whereas no effect on the course of the disease was observed in the Rd11 rats, and variable results were obtained in the Rd 10 animals. Infiltration of mononuclear cells and leakage of 125I-albumin into the peripheral nerve was reduced in the Rd9 rats killed during the suppressed period. The delay in the onset of early clinical signs in the Rd9 rats correlated well with the time-course of serotonin depletion as reflected by levels in the peripheral blood of reserpine-treated normal animals. Although a role for histamine could not be demonstrated, the results suggest that other vasoactive amines are involved in the reaction. These results, therefore, would support the hypothesis that vasoactive amines play a role in the perivascular transit of inflammatory cells in EAN.

Expression of carbonic anhydrase II mRNA and protein in oligodendrocytes during toxic demyelination in the young adult mouse

The aim of this study was to identify events that might take place in oligodendrocytes early in the process of demyelination, i.e., before the occurrence of massive loss of myelin. It was considered important to focus on demyelination and remyelination in young adults, in whose brains there would be relatively few juvenile glial precursor cells. CAII mRNA and protein were used to monitor changes in oligodendrocytes during cuprizone intoxication in the mice. After four or eight weeks of cuprizone feeding CAII message became less plentiful in oligodendrocyte processes. Two days after removal of cuprizone CAII message had appeared in those cell processes. Four or eight weeks after beginning cuprizone feeding CAII protein had decreased∼25% in forebrain homogenates. The loss of CAII protein was reversible after four weeks on cuprizone, but not after eight weeks. After four weeks of cuprizone feeding the numbers of CAII mRNA-prositive oligodendrocytes had decreased by ∼50%m and after eight weeks, by ∼80%. By 12 weeks, however, the number of oligodendrocytes expressing CAII mRNA had spontaneously returned to normal levels. Before eight weeks of cuprizone feeding, loss of myelinated tracts in the corpus striatum was reversible. Demyelination appreared to become irreversible after nine weeks of intoxication, although expression of CAII mRNA remained reversible. The results suggest that in the brain of the young adult, oligodendrocytes expressing message for CAII can be generated spontaneously shortly before demyelination becomes irreversible, and can survive and continue to express CAII mRNA but not CAII protein.

Acetyl-CoA carboxylase in rat brain. II. Immunocytochemical localization

Acetyl-CoA carboxylase (ACC) catalyzes the first and, possibly, the rate-limiting step in fatty acid biosynthesis. Because oligodendrocytes must synthesize large amounts of lipid during myelination, the hypothesis was proposed that ACC might be localized in cells of that type. In sections from the brains of 12-day-old rats, ACC immunostaining was observed in glial cells in white matter and gray matter. These cells resembled carbonic anhydrase-positive oligodendrocytes at mature and immature stages of their development. Cells resembling typical oligodendrocytes were also ACC-positive in white matter from the forebrains and brainstems of 15-17 day-old-rats. In both the gray matter and the white matter of 21-day-old rats there were intensely ACC-positive cells that strongly resembled oligodendrocytes. Oligodendrocytes in the brains of adult rats also were ACC-positive. While recognizing that some ACC must be present at lower levels in other types of cells and at all ages, it was concluded that the present findings are consistent with its primary locus as the oligodendrocytes, particularly during myelination. Further, enrichment of ACC and carbonic anhydrase in the same type of cell suggested that carbonic anhydrase might serve in providing a substrate, bicarbonate, to be utilized by ACC.