Cellular localization and regulation of glutamine synthetase in primary cultures of brain cells from newborn mice

Glutamine synthetase activity during mouse brain development

The specific activity of glutamine synthetase (GS) in mouse brain was 2-fold higher in the olfactory bulbs than in other regions. After birth, the specific activity of GS increased more rapidly in medulla oblongata and in olfactory bulbs, than in cerebral and cerebellar cortex. The activity of GS in primary cultures of brain hemispheres increased more slowly than in homogenates of whole brains. However, when astroblasts were treated in vitro with glucocorticoids or mouse brain extracts, GS activity reached 4 times the level measured in the homogenate of an adult mouse brain. We conclude that levels of GS activity may relate to the maturation of astrocytes, and propose that GS may be used as a marker of astrocytic maturation.

Metabolic fate of 14C-labeled glutamate in astrocytes in primary cultures

The metabolic fate of L-[U-14C]- and L-[1-14C]glutamate was studied in primary cultures of mouse astrocytes. Conversion of the uniformly labeled compound to glutamine and aspartate was followed by determination of specific activities after dansylation with [3H]dansyl chloride and subsequent thin layer chromatography of the dansylated amino acids. Metabolic fluxes were calculated from the alterations of specific activities and the pool sizes, which were likewise measured by a dansylation method. Formation of 14CO2 from [1-14C]glutamate was determined by the trapping of CO2 in hyamine hydroxide in a gas-tight chamber, which is, in the known absence of glutamate decarboxylase activity in the cultured astrocytes, an unequivocal expression of the metabolic flux via alpha-ketoglutarate to CO2 and succinyl-CoA. The metabolic fluxes determined by these procedures amounted to 2.4 nmol/min/mg protein for glutamine synthesis, 1.1 nmol/min/mg protein for aspartate production, and 4.1 nmol/min/mg protein for formation and subsequent decarboxylation of alpha-ketoglutarate. The latter process was unaffected by virtually complete inhibition of glutamate-oxaloacetic transaminase with aminooxyacetic acid, indicating that the formation of alpha-ketoglutarate occurs as an oxidative deamination rather than as a transamination. This suggests that the formation of alpha-ketoglutarate from glutamate represents a net degradation, not an isotopic exchange.

Stimulation of proliferation and maturation of rat astroblasts in serum-free culture by an astroglial growth factor

Astroblasts from brain hemispheres of newborn rats were cultivated for 5 days in a complete medium containing 10% fetal calf serum. Cultures were then grown in basal medium in absence of serum. In these conditions, after 15 days only a few cells remained. When the basal culture medium was supplemented with insulin and transferrin (IT medium) the number of the remaining cells was twice higher. However, it was still lower than that found in the 5-day cultures before serum was removed. Thus the IT medium does not allow a good survival of the cells. Addition to the medium of an astroglial growth factor (AGF) from bovine brain soluble extract led to a tenfold increase of the cell number compared to cultures maintained in the basal medium alone. Effects of the factor on cell survival and on proliferation were examined independently. It was found that the factor did not greatly increase the survival of the cells but that it stimulated the proliferation. Addition of the growth factor to the IT medium elicited an increase of the level of S-100 protein and of the activity of glutamine synthetase, two proteins which are specific to astroglial cells in the central nervous system. These results indicate that the astroglial brain factor allows growth and maturation of the rat astroblasts maintained in serum-free medium.