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Neurotrophin-induced neuroprotection against apoptosis was investigated using immature cultured cerebellar granule cells (CGC) from newborn rat pups. Apoptotic cell death induced by treatment with cytosine arabinoside (AraC) was confirmed by DNA fragmentation and quantified by cell survival assays. AraC was most effective in inducing apoptosis when added to CGC on the day of culture preparation, while less or no effect was observed when added at 24 or 48h after plating, respectively. Pretreatment of CGC cultures for 24h with brain-derived neurotrophic factor (BDNF) or neurotrophin-4 (NT-4), but not neurotrophin-3 (NT-3), robustly protected against AraC neurotoxicity. K252a, an inhibitor of the tropomyosin-related kinase (Trk) tyrosine kinase receptor family which showed no toxicity by itself, blocked BDNF protection of AraC-induced apoptosis in a concentration-dependent manner. Neither protein kinase C activation nor inhibition mimicked or affected BDNF protection against AraC neurotoxicity. BDNF, but not NT-3, treatment of immature CGC caused a marked, but transient activation of Akt through phosphatidylinositol (PI) 3-kinase. The neuroprotective effects of BDNF were suppressed by pretreatment with LY 294002 (a PI 3-kinase inhibitor). BDNF neuroprotection was also preceded by activation of mitogen activated protein kinase (MAPK) and suppressed by two MAPK/ERK (MEK)-selective inhibitors, PD 98059 and U-0126. Moreover, inhibitors of PI 3-kinase and MEK potentiated AraC-induced neurotoxicity. These results show that neurotrophins protect against AraC-induced apoptosis, at least in part, through TrkB-mediated activation of the PI 3-kinase/Akt and MEK signaling pathways.

In rat cerebellar granule cells, glutamate induced rapid activation of c-Jun N-terminal kinase (JNK) and p38 kinase to phosphorylate c-Jun (at Ser63) and p53 (at Ser15), respectively, and a subsequent marked increase in activator protein-1 (AP-1) binding that preceded apoptotic death. These glutamate-induced effects and apoptosis could largely be prevented by long-term (7 days) pretreatment with 0.5-2 mm lithium, an antibipolar drug. Glutamate's actions could also be prevented by known blockers of this pathway, MK-801 (an NMDA receptor blocker), SB 203580 (a p38 kinase inhibitor) and curcumin (an AP-1 binding inhibitor). The concentration- and time-dependent suppression of glutamate's effects by lithium and curcumin correlated well with their neuroprotective effects. These results suggest a prominent role of JNK and p38, as well as their downstream AP-1 binding activation and p53 phosphorylation in mediating glutamate excitotoxicity. Moreover, the neuroprotective effects of lithium are mediated, at least in part, by suppressing NMDA receptor-mediated activation of the mitogen-activated protein kinase pathway.

The cyclic AMP response element binding protein (CREB) has major roles in mediating adaptive responses at glutamatergic synapses and in the neuroprotective effects of neurotrophins. CREB has been implicated as a potential mediator of antidepressant actions. In vitro, chronic lithium treatment has been shown to promote neuronal cell survival. In the present study, we have used cultures of cerebellar granule neurons to analyze the effects of acute and chronic lithium treatment on the response to toxic concentrations of glutamate. Such concentrations of glutamate decrease the phosphorylation of CREB at serine(133) in an N-methyl-D-aspartate (NMDA) receptor-dependent manner. Chronic, but not acute, lithium treatment suppresses glutamate-induced decreases in phosphorylated CREB, and transfection studies indicate that chronic lithium, in the presence of a glutamate stimulus, markedly increases CRE-driven gene expression. Experiments with selected pharmacological reagents indicate that the glutamate-induced decreases in phosphorylated CREB are regulated primarily by protein phosphatase 1. Chronic lithium treatment not only decreases protein phosphatase 1 activity under these circumstances, but also augments glutamate-induced increases in MEK activity. PD 98059, a MEK inhibitor, prevents chronic lithium treatment from increasing phosphorylated CREB levels in glutamate-treated neurons. We conclude from these results that chronic lithium treatment is permissive for maintaining higher phosphorylated CREB levels in the presence of glutamate in part by decreasing protein phosphatase 1 activity and in part by increasing MEK activity. Higher levels of phosphorylated CREB and CRE-responsive genes such as bcl-2 may be responsible for lithium's reported effects on neuronal survival.

This report describes a modulatory action of lithium and glutamate on the activity of serine/threonine kinase Akt-1. Lithium is most commonly used to treat bipolar disorder, but the mechanism of its therapeutic action remains unknown. We have recently demonstrated that lithium protects against glutamate-induced excitotoxicity in cultured brain neurons and in an animal model of cerebral ischemia. This study was undertaken to investigate the role of Akt-1, activated by the phosphatidylinositol 3-kinase (PI 3-K) signaling pathway, in mediating glutamate excitotoxicity and lithium protection in cerebellar granule cells. High levels of phosphorylation and activity of Akt-1 were detected in cerebellar neurons cultured in the presence of serum. Protracted treatment with selective PI 3-K inhibitors, wortmannin and LY294002, abolished Akt-1 activity and induced neuronal death that could be reduced by long-term lithium pretreatment. Exposure of cells to glutamate induced a rapid and reversible loss of Akt-1 phosphorylation and kinase activity. These effects were closely correlated with excitotoxicity and caspase 3 activation and were prevented by phosphatase inhibitors, okadaic acid and caliculin A. Long-term lithium pretreatment suppressed glutamate-induced loss of Akt-1 activity and accelerated its recovery toward the control levels. Lithium treatment alone induced rapid increase in PI 3-K activity, and Akt-1 phosphorylation with accompanying kinase activation, which was blocked by PI 3-K inhibitors. Lithium also increased the phosphorylation of glycogen synthase kinase-3 (GSK-3), a downstream physiological target of Akt. Thus, modulation of Akt-1 activity appears to play a key role in the mechanism of glutamate excitotoxicity and lithium neuroprotection.

Previous studies from our laboratory have demonstrated that stimulation of 5-hydroxytryptamine2A receptors in rat cerebellar granule cells produces an increase in the levels of 5-hydroxytryptamine2A receptor messenger RNA and binding sites, and that this up-regulation requires de novo RNA and protein synthesis. Here we showed that up-regulation of 5-hydroxytryptamine2A receptor binding sites induced by stimulation with the 5-hydroxytryptamine2A/2C receptor agonist, (+/-)-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), is associated with an increase in the 5-hydroxytryptamine2A receptor transcription rate. To examine the possible role of transcriptional activation in DOI-induced 5-hydroxytryptamine2A receptor up-regulation, we studied the effects of DOI on transcription factor binding to activator protein-1 and cyclic AMP-responsive element (CRE) DNA consensus sequences. We found that DOI induces a time-dependent increase in activator protein-1 and CRE transcription factor binding activity, which is blocked by 5-hydroxytryptamine2A receptor antagonists. Similar to 5-hydroxytryptamine2A receptor up-regulation, DOI-induced activator protein-1 binding is suppressed by inhibitors of calmodulin and Ca2+/calmodulin-dependent kinases. The increased activator protein-1 binding is effectively competed by excessive activator protein-1 and CRE sequences as well as endogenous activator protein-1-like sequences present in the rat 5-hydroxytryptamine2A receptor gene. Supershift assays revealed that cAMP-responsive element-binding protein (CREB) and Jun D are common components of both activator protein-1 and CRE binding complexes. DOI also increased the level of phospho-CREB in a time-dependent manner. The binding of phospho-CREB transcription factor to the activator protein-1 site suggests that CREB may modulate the transcription of genes that contain activator protein-1 but lack CRE site in their promoters, through interaction with the activator protein-1 site. The rat 5-hydroxytryptamine2A receptor up-regulation may involve such a mechanism.

We have previously shown that cytosine arabinoside (AraC)-induced apoptosis of cerebellar granule cells (CGCs) results in an increase of a 38-kDa band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, identified as glyceraldehyde-3-phosphate dehydrogenase (GAPDH; EC 1.2.1.12). Antisense oligonucleotides to GAPDH mRNA afford acutely plated CGCs significant protection against AraC-induced apoptosis. We used differential centrifugation to examine which subcellular components are affected. Treated and untreated cells were sonicated in 0.32 M sucrose and sequentially centrifuged at 1,000, 20,000, and 200,000 g, to obtain crude nuclear, mitochondrial, microsomal, and cytosolic fractions. Western blotting showed that the levels of GAPDH protein were markedly increased in the 1,000- and 20,000-g pellets. The levels in the cytosolic supernatant were decreased dramatically by AraC in acutely plated CGCs but not in cells 24 h after plating. It is noteworthy that although GAPDH protein in the pellet fractions increased, the dehydrogenase activity of GAPDH decreased. Two other dehydrogenases, lactate dehydrogenase (EC 1.1.1.27) and glucose-6-phosphate dehydrogenase (EC 1.1.1.49), were not similarly affected, suggesting that the effect was GAPDH specific. These observations suggest that GAPDH levels change in specific organelles during apoptosis for reasons that are separate from its function as a glycolytic enzyme. The accumulation of GAPDH protein in specific subcellular loci may play a role in neuronal apoptosis.