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Rat cerebellar granule cell culture is widely used as a model to study factors that control neuronal differentiation and death (e.g. excitotoxicity). However, a main drawback of this model is its dependence on depolarizing culture condition (25 mM potassium). In addition, it is quite expensive to maintain and requires animal facilities. Here we report that cerebellar granule neuron cultures from chicken may be used as an alternative model to study excitotoxicity. Surprisingly, fetal chicken cells may be grown in a physiological potassium concentration (5 mM potassium). They develop excitotoxicity rapidly in culture (fully developed at 3 days in vitro), and respond to glutamate excitotoxicity similar to rat cultures (ROS production and activation of caspase-3).

Transcription factor NGFI-B initiates apoptosis when allowed to translocate to mitochondria. Retinoid-X receptor (RXR), another member of the nuclear receptor family, regulates NGFI-B signaling through heterodimerization and nuclear export. Growth factor EGF activates ERK2, which phosphorylates NGFI-B and determines if NGFI-B is allowed to translocate to mitochondria. In the present study, EGF treatment resulted in an increased nuclear import of NGFI-B. Likewise, active ERK2 resulted in a preferential nuclear localization of NGFI-B. When coexpressed with RXR the nuclear import and nuclear localization induced by active ERK2 were strongly reduced. In the presence of its ligand 9-cis-retinoic acid, RXR no longer inhibited ERK2-induced nuclear import. Thus, RXR serves a permissive role for ERK2-mediated nuclear accumulation of NGFI-B. This finding represents a novel crosstalk between ERK2 and RXR signaling pathways, and explains how two independent inhibitors of apoptosis (EGF and 9-cis-retinoic acid) may cooperate to regulate nuclear targeting of apoptosis inducer NGFI-B.

Many recent reports on internucleosomal DNA fragments have appeared, however, little is known about the mechanisms of the generation of their upstream high molecular weight (HMW) fragments. Caspases are a family of proteases with important functions in the execution of apoptotic cell death. The caspase-sensitivity of the formation of HMW fragments was therefore investigated using a specific caspase-3 inhibitor (Ac-DEVD-cmk) and a general caspase inhibitor (boc-D-fmk). Apoptosis inducing factor (AIF) can translocate to the nucleus and generate HMW fragments independently of caspase. Cultures of cerebellar granule neurons (CGNs) were therefore exposed to glutamate (100 micro M) or deprived of potassium and serum to induce apoptosis, or treated with a high concentration of calcium ionophore A23187 (1 micro M) to induce necrosis. Fragmentation of DNA into two classes of HMW fragments (680 and 50-300 kbp) was observed after treatment with glutamate or A23187. Traces of approximately 50-kbp fragments were detectable after the K(+)/serum-deprivation. The amount of680-kbp HMW fragments increased (i.e. their further degradation was inhibited) and cell death was reduced in the presence of Ac-DEVD-cmk or boc-D-fmk following glutamate treatment. Only boc-D-fmk treatment resulted in a similar accumulation of680-kbp HMW fragments and reduced cell death after K(+)/serum-deprivation. No such changes were observed with caspase inhibitors after A23187 treatment. AIF redistribution was observed following glutamate treatment and K(+)/serum-deprivation. Thus, even in a simple cell culture of CGNs, HMW fragments are formed by diverse mechanisms: the degradation of DNA may be sensitive to different caspases or be caspase and AIF independent.

Since dexamethasone may elevate the Ca2+ influx through NMDA receptors, we have investigated mechanisms of dexamethasone toxicity in rat cerebellar granule neurons. Dexamethasone concentrations over 0.1 microM induced cell death that reached about 20% of the death induced by glutamate. Dexamethasone-induced cell death was reduced by more than 80% by the mineralocorticoid antagonist RU 28318 or the NMDA receptor antagonists MK 801 and CGP 39551, whereas RU 28318 rescued only approximately 30% of cells treated with glutamate, indicating that dexamethasone requires NMDA receptors to induce acute neuronal toxicity and that a fraction of the neurons showed this toxicity. Mg2+ reduced the cell death induced by glutamate at potassium concentrations of 1 mM and 5 mM, but not at 25 mM. In contrast, cell death induced by dexamethasone was not significantly reduced by Mg2+ in any of the potassium concentrations. Both glutamate and dexamethasone induced toxicity with translocation of the apoptosis inducer NGFI-B to the mitochondria seen after 30 min-2 h concomitant with activation of apoptosis inducing factor (AIF) and caspase-3. In conclusion, dexamethasone induces a rapid toxicity which is blocked by NMDA receptor antagonists other than Mg2+, and involves mitochondrial apoptosis inducer NGFI-B.