Multipotent stem cells from the mouse basal forebrain contribute GABAergic neurons and oligodendrocytes to the cerebral cortex during embryogenesis

During CNS development, cell migrations play an important role, adding to the cellular complexity of different regions. Earlier studies have shown a robust migration of cells from basal forebrain into the overlying dorsal forebrain during the embryonic period. These immigrant cells include GABAergic neurons that populate the cerebral cortex and hippocampus. In this study we have examined the fate of other basal forebrain cells that migrate into the dorsal forebrain, identifying basal cells using an antibody that recognizes both early (dlx1/2) and late (dlx 5/6) members of the dlx homeobox gene family. We found that a subpopulation of cortical and hippocampal oligodendrocytes are also ventral-derived. We traced the origin of these cells to basal multipotent stem cells capable of generating both GABAergic neurons and oligodendrocytes. A clonal analysis showed that basal forebrain stem cells produce significantly more GABAergic neurons than dorsal forebrain stem cells from the same embryonic age. Moreover, stem cell clones from basal forebrain are significantly more likely to contain both GABAergic neurons and oligodendrocytes than those from dorsal. This indicates that forebrain stem cells are regionally specified. Whereas dlx expression was not detected within basal stem cells growing in culture, these cells produced dlx-positive products that are capable of migration. These data indicate that the developing cerebral cortex incorporates both neuronal and glial products of basal forebrain and suggest that these immigrant cells arise from a common progenitor, a dlx-negative basal forebrain stem cell.

Transmitter-induced calcium responses differ in astrocytes acutely isolated from rat brain and in culture

Glial fibrillary acid protein (GFAP)-positive astrocytes isolated from the cerebral cortices of 3-10-day-old rats frequently showed increased intracellular Ca2+ concentration responses to L-glutamate and glutamate analogues. However, few of the acutely isolated cells responded to ATP, and no such cells responded to serotonin [5-hydroxytryptamine (5-HT)]. The same cell that failed to respond to ATP or 5-HT often responded to glutamate. Culturing acutely isolated cells in media containing horse serum decreased Ca2+ responses to glutamate but increased the responses to ATP and induced responses to 5-HT. In primary cultures prepared from the cerebral cortices of 1-day-old rats and cultured in horse serum, fewer of the cells responded to glutamate, but almost all cells responded to ATP and 5-HT. The lack of or limited response to, 5-HT or ATP in the acutely isolated cells seems unlikely to be due to selective damage to the respective receptors because acutely isolated GFAP-negative cells showed responses to ATP, several different proteases and mechanical dissociation yielded cells that also responded to glutamate but not to ATP, and exposure of primary cultures to papain did not abolish Ca2+ responses to several transmitters. The responses of the acutely isolated cells to glutamate but limited or lack of responses to ATP and 5-HT also correspond to what has been seen so far for astrocytes in situ. Thus, the present studies provide direct evidence that some of the receptors seen in primary astrocyte cultures may reflect a response to culture conditions and that, in the context of the relevant information so far available, acutely isolated astrocytes seem to reflect better the in vivo state.

Astrocytic swelling due to hypotonic or high K+ medium causes inhibition of glutamate and aspartate uptake and increases their release

Astrocytic swelling occurs readily in ischemia and traumatic brain injury (TBI) as part of the cytotoxic or cellular edema response. Ischemia is known to produce large extracellular increases in both [K+] and excitatory amino acids (EAA) in vivo, and astrocytic swelling in vitro leads to marked release of EAA. In this study we compared the effect of swelling due to hypotonic media and high K+ medium on the uptake and release of EAA by rat primary astrocyte cultures in vitro. In both cases, there was a significant inhibition of uptake of [3H]L-glutamate and [3H]D-aspartate, and increased release of preloaded [3H]D-aspartate. The kinetics of the increased efflux was very different in response to hypotonic or high K+ media. In hypotonic medium there was a rapid initial release followed by a decline in the rate of release over time. This release was independent of whether Na+ was present. Upon exposure to high K+ medium there was a slow progressive increase in release of [3H]D-aspartate, which never showed any subsequent decline until the media was returned to normal [K+]. In high K+ media there was also an initial transient increase in [3H]D-aspartate release, which we attribute to reversal of the amino acid uptake system. The increased release due to hypotonic medium was not affected by a drop in temperature from 37 to 26 degrees C, while the increased release due to high K+ medium was completely inhibited.(ABSTRACT TRUNCATED AT 250 WORDS)

Astrocytes as mediators of methylmercury neurotoxicity: effects on D-aspartate and serotonin uptake

In this study we address the effects of methylmercuric chloride (MeHgCl), a metal that is preferentially sequestered in astrocytes, on 5-HT and glutamate/aspartate uptake by rat primary astrocyte cultures. Quantitative autoradiography (ARG) combined with glial acidic fibrillary protein (GFAP) immunocytochemistry, as well as intact-cell (bulk) measurements of radiolabel uptake of these neurotransmitters were performed in 7- and 21-day-old primary astrocyte cultures. MeHg (10 microM for 30 min) treatment of astrocytes (21 days in culture) significantly inhibited the Na(+)-dependent and fluoxetine-sensitive [3H]5-HT uptake. D-aspartate uptake in 7- and 21-day-old cultures was even more sensitive to MeHg, leading to > 99% inhibition of D-aspartate uptake by astrocytes (30 min; 10 microM MeHg). These results imply that the Na(+)-dependent and fluoxetine-sensitive 5-HT uptake, as well as the Na(+)-dependent L-glutamate/D-aspartate uptake systems in primary astrocyte cultures are sensitive to low concentrations of MeHg. Since astrocytic removal of glutamate (and aspartate) and 5-HT from the extracellular space in situ is crucial to the maintenance of chemical homeostasis, MeHg-induced uptake inhibition of 5-HT and aspartate could have cytotoxic effects on neighboring neurons.

Methylmercury uptake in rat primary astrocyte cultures: the role of the neutral amino acid transport system

The significance of the dense labeling pattern of methylmercury (MeHg) over astrocytes in areas of damaged cortex remains obscure, and the extent to which individual neurons are altered by MeHg accumulation in astrocytes is unknown. As a first step in understanding the relationship between the astrocyte and the mechanisms of MeHg's neurotoxicity, studies were directed at how MeHg is transported into cultured astrocytes. Uptake of [203Hg]MeHg in primary astrocyte cultures from neonatal rat cerebral cortex following incubations with MeHgCl conformed to a simple diffusion process. Uptake of [203Hg]MeHg by astrocytes exhibited the kinetic criteria of a specific transport system when added to the media as the L-cysteine conjugate. Saturation kinetics, substrate specificity and inhibition, and trans-stimulation were demonstrated in the presence of this SH-containing amino acid. Cysteine-mediated uptake of MeHg was inhibited by the coadministration of L-methionine, and 2-aminobicyclo-[2,2,1]-heptane-2-carboxylic acid. 2-Methylaminoisobutyric acid was ineffective in inhibiting the uptake of the MeHg-cysteine conjugate. Preloading of the astrocytes with glutamate was moderately effective in trans-stimulating the uptake of MeHg-cysteine conjugates, while in the absence of cysteine, uptake of [203Hg]MeHg was unchanged. These results indicate the presence in astrocytes of a neutral amino acid carrier transport System L, capable of selectively mediating cysteine-MeHg uptake. The substrate specificity and high affinity of this transport system resemble the properties of the System L neutral amino acid transport across the blood-brain barrier in the rat. Cellular uptake of MeHg-cysteine conjugates was not inhibited by preincubation of astrocytes with 100 microM N-ethylmaleimide or NaF.(ABSTRACT TRUNCATED AT 250 WORDS)