Glutathione S-transferase isoenzymes in rat brain neurons and glia

Abstract

The glutathione S-transferases (GSTs) constitute a family of cytosolic isoenzymes and a structurally unrelated microsomal enzyme that is involved in the detoxication of electrophilic xenobiotics. These enzymes also participate in the intracellular binding and transport of a broad range of lipophilic compounds including bilirubin, and hormones such as the glucocorticoids and thyroid hormones. The present investigation demonstrates that GSTs are present in neurons of the brainstem, forebrain, and cerebellum. An isoenzyme- specific distribution of GSTs was found in cytoplasm, nuclei, and nucleoli. The regional and cellular distribution of cytosolic GSTs in the brain was studied by immunohistochemistry, spectrophotometric enzyme assay, and reverse-phase HPLC. Polyclonal antibody against microsomal GST was strongly reactive with Purkinje cells throughout the cerebellar cortex, and with neurons in the brainstem and hippocampus. Nuclei of Purkinje cells and of neurons in the brainstem, hippocampus, and cerebral cortex were immunopositive for α-class GST 1-1 (Y(a)Y(a)), whereas α-class GST 2-2 (Y(c)Y(c)) antibody was consistently immunoreactive with the nucleolus, but not with the nucleus or soma. All α-class GST antibodies studied were reactive, to various degrees, with astrocytes and choroid plexus; however, ependymal cells of the subventricular zones were immunonegative. α-class GST 8-8 (Y(k)Y(k)) immunoreactivity was specifically localized to endothelial cells and/or astrocytic end feet associated with blood vessels. Reverse- phase HPLC indicated that there were also substantial regional differences in the pattern of α-, μ-, and π-class GST subunit expression. For example, the thalamus/hypothalamus had the highest GST activity and greatest concentration of total GST protein and μ-class GST subunit 6 (Y(b3)), whereas the brainstem had the greatest concentration of π-class GST subunit (Y(p)). This regional variation in GST expression may be reflective of regional differences in cell populations. In cerebellar cortex, the concentration of μ-class GST subunit 4 (Y(b2)) was greatest in the flocculus and lowest in the vermis. This is of clinical interest because the pattern of expression of μ-class GST subunit 4 (Y(b2)) in the cerebellum coincides with the known regional susceptibility of this structure to degeneration after exposure to toxic or metabolic insults. The vermis is most susceptible to these insults, whereas the lateral lobes and flocculus are most resistant. The nuclear localization of α-class GSTs in neurons, the cytoplasmic localization of microsomal GST in neurons, and the association between concentration of μ-class GST subunit 4 (Y(b2)) and resistance of neurons to toxic events in the cerebellar cortex suggest that GSTs may confer protection against exogenous and/or endogenous neurotoxic metabolites.