Preliminary evaluation of an in vitro test for assessment of excitotoxicity by measurement of early gene (c-fos mRNA) levels

Transcriptomic responses in mouse brain exposed to chronic excess of the neurotransmitter glutamate

Background

Increases during aging in extracellular levels of glutamate (Glu), the major excitatory neurotransmitter in the brain, may be linked to chronic neurodegenerative diseases. Little is known about the molecular responses of neurons to chronic, moderate increases in Glu levels. Genome-wide gene expression in brain hippocampus was examined in a unique transgenic (Tg) mouse model that exhibits moderate Glu hyperactivity throughout the lifespan, the neuronal Glutamate dehydrogenase (Glud1) mouse, and littermate 9 month-old wild type mice.

Results

Integrated bioinformatic analyses on transcriptomic data were used to identify bio-functions, pathways and gene networks underlying neuronal responses to increased Glu synaptic release. Bio-functions and pathways up-regulated in Tg mice were those associated with oxidative stress, cell injury, inflammation, nervous system development, neuronal growth, and synaptic transmission. Increased gene expression in these functions and pathways indicated apparent compensatory responses offering protection against stress, promoting growth of neuronal processes (neurites) and re-establishment of synapses. The transcription of a key gene in the neurite growth network, the kinase Ptk2b, was significantly up-regulated in Tg mice as was the activated (phosphorylated) form of the protein. In addition to genes related to neurite growth and synaptic development, those associated with neuronal vesicle trafficking in the Huntington's disease signalling pathway, were also up-regulated.

Conclusions

This is the first study attempting to define neuronal gene expression patterns in response to chronic, endogenous Glu hyperactivity at brain synapses. The patterns observed were characterized by a combination of responses to stress and stimulation of nerve growth, intracellular transport and recovery.

Improvements in an in-vitro assay for excitotoxicity by measurement of early gene (c-fos mRNA) levels

Quantitative, real-time reverse transcription-polymerase chain reaction (RT-PCR) measurements were made to investigate the levels of c-fos mRNA as one measure of the expression of the c-fos gene. Exposure of mouse cerebellar granule cells to excitotoxic concentrations of glutamate (Glu) and aspartate (Asp) led to a changed time profile for mRNA expression, from a transient c-fos expression at 15-30 min to a delayed, elevated and sustained expression at later time points which was prevented by selective antagonism of the NMDA receptor but not of the AMPA/kainate receptor demonstrating that this c-fos induction was mediated through the specific activation of the NMDA Glu receptor subtype. The question as to whether c-fos expression changes could be used to predict excitotoxicity was addressed by testing the c-fos response of the cultures to several compounds, at low (and therefore non-toxic) and high (toxic) concentrations at two suitable time-points of exposure (30 and 240 min), in the presence and absence of Glu receptor antagonists. The compounds were divided into four groups, excitotoxins, neurotoxic but non-excitotoxic compounds, neuroactive but non-toxic compounds, and compounds that were toxic to other target organelles. The results of this study, using real-time RT-PCR, support the proposal that c-fos mRNA can be used as a specific biomarker of excitotoxicity and moreover encourage further studies to employ this highly sensitive, quantifiable and reproducible technique in a high throughput screen, with minimal use of animals for primary culture set-up. Furthermore, this test has the potential for application in screening newly-designed excitatory amino acid receptor antagonists in the search for clinically relevant drugs to treat a variety of neuropathologies.

Differential mechanisms of glutamate-stimulated perturbations in the kinetics of c-fos mRNA induction are associated with maturation of cerebellar granule cells in primary culture

In further exploring proposals for the measurement of early gene (c-fos mRNA) levels as a predictive index for in vitro excitotoxicity, this study, using immature (2 days in vitro) cultures of mouse cerebellar granule cells as an experimental model system, was undertaken to determine the effect of glutamate (Glu) i) in stimulating increases in intracellular free-calcium ([Ca2+]), ii) on cell viability and iii) on induction of steady-state c-fos mRNA levels. In parallel experiments the action of agents (viz. 55 mM KCl and the calcium ionophore, A23187) that mediate Ca2+ entry into cells via different routes was also evaluated. Glu was unable to induce excitotoxicity in granule cells at this stage of development in culture, but did stimulate a concentration-dependent and marked increase in [Ca2+], levels while also mediating a dramatic concentration-dependent perturbation in the kinetics of c-fos mRNA induction that appeared to arise solely from NMDA receptor-mediated Ca2+ influx. The results are presented in comparison to the actions of KCI and A23187 and considered in relation to earlier studies undertaken using mature (7 days in vitro) cultures of cerebellar granule cells.

Possible role for the FosB/JunD AP-1 transcription factor complex in glutamate-mediated excitotoxicity in cultured cerebellar granule cells

The potent excitatory and neurotoxic actions of glutamate are known to influence the expression of a variety of genes, including those encoding the AP-1 transcription factor, which comprises proteins belonging to the Fos and Jun families. However, the precise role of Fos- and Jun-like transcription factors in these events remains elusive. Here we demonstrate, using primary cultures of mouse brain cerebellar granule cells as an in vitro model system, a possible involvement of the FosB/JunD heterodimer in excitotoxicity. Granule cells were grown for either 2 or 7 days in vitro (DIV) before exposure to varying concentrations (1-3000 microM) of the excitotoxin glutamate. In 7-DIV cells, glutamate induced a concentration-dependent neuronal death, whereas, in 2-DIV cells, no glutamate-induced neuronal damage was seen. We were particularly interested in comparing the protein composition of the AP-1 transcription factor complex in cells exposed to excitotoxic and to nontoxic conditions. AP-1 DNA binding activity was demonstrated by gel shift analysis in nuclear extracts derived from 7-DIV cells following exposure to either a nontoxic (10 microM) or an excitotoxic (250 microM) dose of glutamate and was similarly observed in extracts of 2-DIV cells exposed to the same levels of glutamate. Gel supershift analysis using antibodies against the different Fos and Jun family members allowed differentiation between AP-1 DNA binding in nuclear extracts as a function of both 1) viability status and 2) the stage of development. Of major significance was the finding that FosB could be detected as a component of AP-1 in 7-DIV cells only under excitotoxic conditions, whereas c-Fos, Fra-2, and JunD proteins were detectable under both excitotoxic and nontoxic conditions in cells of this age. In 2-DIV cells (in which glutamate is nontoxic), AP-1 comprised combinations of only Fra-1, Fra-2, c-Jun, and JunD. Because Fos family members are unable to form homodimers, this finding raises the possibility that the FosB/JunD heterodimer may have special significance in the mechanism of excitotoxic neuronal death.