Apoptosis, also referred to as programmed cell death, is a process in which a cell dies by activation of an intrinsic genetic program. Apoptosis plays an important role in the development and aging processes in the central nervous system (CNS). Abnormality in apoptosis has been linked to pathogenesis of neurodegenerative diseases and implicated in neuropsychiatric disorders. We have used primary cultures of rat CNS neurons and neurally related cell lines as a model to study molecular mechanisms underlying neuronal apoptosis. We found that glyceraldehyde-3-phosphaate dehydrogenase (GAPDH), a glycolytic enzyme with multiple nonglycolytic functions, is overexpressed during apoptosis induced by aging of rat cerebellar granule cells in culture. Antisense oligonucleotides to GAPDH block GAPDH overexpression and effectively delay age-induced apoptosis of these cerebellar neurons. These results provide the first evidence for a role of GAPDH in neuronal apoptosis. By the same criteria, we found that GAPDH is involved in age-induced apoptosis of rat cerebral cortical neurons and apoptosis of cerebellar granule cells induced by extracellular potassium deprivation and exposure to cytosine arabinoside (AraC). Preliminary results also show that GAPDH antisense oligonucleotides block apoptosis of pheochromocytoma PC-12 cells induced by withdrawal of serum and nerve growth factor. In the case of AraC-induced apoptosis, we found that the cell death occurs within a narrow frame of approximately 40 hours after plating and is robustly protected by neurotrophins, BDNF, and NT 4/5 but not NT-3. The apoptosis is also associated with an increased expression of two death genes, p53 and Bax, preceding the overexpression of GAPDH. Moreover, an antisense oligonucleotide to p53 reduces GAPDH induction and protects against AraC-induced apoptosis, suggesting that GAPDH overexpression depends on p53 expression. AraC-induced GAPDH overexpression is predominantly accumulated in the nucleus assessed by subcellular fractionation study ad electromicroscopic immunohistochemistry. Translocation of GAPDH to the nucleus is not parallel with an increase in GAPDH glycolytic activity, suggesting that the nonglycolytic function(s) participates in neuronal apoptosis. In the case of potassium deficiency-induced death of cerebellar granule cells, which involves both apoptosis and necrosis, GAPDH is accumulated in both nuclear and mitochondrial fractions. Recently, GAPDH has been shown to bind specifically to gene products of degenerative diseases such as Alzheimer's disease, Huntington's disease, DRPLA, and spinocerebellar ataxia. Our results suggest that such specific binding could be the results of overexpression of GAPDH in CNS neurons, which in turn induces neuronal apoptosis and ultimate neurodegeneration. Moreover, the ability of GAPDH antisense oligonucleotides to rescue neurons from undergoing apoptotic cell death also suggests that such oligonucleotides may be useful for treating nerodegenerative diseases. We also found that amlodipine, a calcium channel blocker with antioxidant activity, is an effective neuroprotectant against age-induced apoptosis of cerebellar granule cells. This property would be related to amlodipine's ability to improve cognitive function in animal behavioral studies. In this context, we also found that ONO-1603, a potential antidementia drug, protects against age-induced apoptotic death of cerebral cortical neurons. Additionally, ONO-1603 and tetrahydroaminoacridine, an FDA-approved antidementia drug, protects cerebellar neurons against low KCl-induced apoptosis. Thus, apoptotic death of CNS neurons in culture could be a valuable tool for screening more effective drugs in the treatment of dementia associated with Alzheimer's disease and AIDS. In an attempt to further elucidate molecular mechanisms underlying apoptosis, we have used cerebellar granule cells treated with AIDS-related neurotoxins, such as 3-OH-kynurenine (3-HK) and quinolinic acid, which are tryptophan metabolites and whose levels are elevated in the brain of AIDS patients. We found that 3-HK induces a robust dose-dependent apoptotic response in the range of 50-1000 muM, while quinolinic acid is ineffective. The 3-HK neurotoxicity is potentiated by superoxide dismutase (SOD) and a cell permeable SOD mimetic, MnTBAP. Both 3-HK-induced and SOD-potentiated neurotoxicities are blocked by catalase, suggestive of hydrogen peroxide formation. Preliminary results show that gp41, a HIV-1 coat protein directly implicated in AIDS pathogenesis, also causes apoptosis of cerebellar neurons. Underlying molecular mechanisms and the role of GAPDH in AIDS-related neuronal death are being explored in cerebellar neurons and other CNS neuronal types.
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