Synthesis of the Antioxidant Glutathione in Neurons: Supply by Astrocytes of CysGly as Precursor for Neuronal Glutathione

Abstract

Deficiency of the antioxidant glutathione in brain appears to be connected with several diseases characterized by neuronal loss. To study neuronal glutathione metabolism and metabolic interactions between neurons and astrocytes in this respect, neuron-rich primary cultures and transient cocultures of neurons and astroglial cells were used. Coincubation of neurons with astroglial cells resulted within 24 hr of incubation in a neuronal glutathione content twice that of neurons incubated in the absence of astroglial cells. In cultured neurons, the availability of cysteine limited the cellular level of glutathione. During a 4 hr incubation in a minimal medium lacking all amino acids except cysteine, the amount of neuronal glutathione was doubled. Besides cysteine, also the dipeptides CysGly and gammaGluCys were able to serve as glutathione precursors and caused a concentration-dependent increase in glutathione content. Concentrations giving half-maximal effects were 5, 5, and 200 microM for cysteine, CysGly, and gammaGluCys, respectively. In the transient cocultures, the astroglia-mediated increase in neuronal glutathione was suppressed by acivicin, an inhibitor of the astroglial ectoenzyme gamma-glutamyl transpeptidase, which generates CysGly from glutathione. These data suggest the following metabolic interaction in glutathione metabolism of brain cells: the ectoenzyme gamma-glutamyl transpeptidase uses as substrate the glutathione released by astrocytes to generate the dipeptide CysGly that is subsequently used by neurons as precursor for glutathione synthesis.

Thimerosal Neurotoxicity is Associated with Glutathione Depletion: Protection with Glutathione Precursors

“…The oxidized form of cysteine (cystine) was protective in astrocytes, but not neurons, consistent with facilitated membrane transport of cystine in astrocytes (Kranich et al, 1998). Neurons depend on glutathione synthesized in the astrocytes and released extracellularly where it is hydrolyzed to cysteinylglycine and cysteine by ectoenzymes to provide neurons with necessary precursors for intracellular gluthathione synthesis (Dringen et al, 1999). In both cell lines, methionine provided no protection against Thimerosal toxicity confirming the inability of either cell type to synthesize cysteine (and glutathione) from methionine.…”

Underestimation of the Pentose–Phosphate Pathway in Intact Primary Neurons as Revealed by Metabolic Flux Analysis

“…The reactions were stopped by adding 0.2 mL of 20%, v/v, HClO 4 through the rubber cap, and flasks were further incubated for 96 hours to allow equilibration of 3 H 2 O between water and the incubation medium, or for 1.5 hours to trap 14 CO 2 into the benzethonium hydroxide. In the preliminary experiments using 3 H 2 O or NaH 14 CO 3 , we observed that the 3 H 2 O equilibrated between both compartments was 28% (hence reflecting an actual 70% of recovery efficiency); the efficiency of 14 …”

Mitochondrial involvement in neurodegenerative diseases

“…Also cultured neurons from the cortex contain considerably less glutathione than astroglial cultures from cortex (51). However, it has been demonstrated that the amount of glutathione in neurons and astroglial cells varies with the brain region from which the cells have been prepared (52), explaining in part the selective vulnerability of certain neuronal populations.…”