Acetyl-CoA metabolism in amprolium-evoked thiamine pyrophosphate deficits in cholinergic SN56 neuroblastoma cells.

Inhibition of pyruvate (PDHC) and ketoglutarate (KDHC) dehydrogenase complexes induced by thiamine pyrophosphate deficits is known cause of disturbances of cholinergic transmission in the brain, yielding clinical symptoms of cognitive, vegetative and motor deficits. However, particular alterations in distribution of key acetylcholine precursor, acetyl-CoA, in the cholinergic neuron compartment of thiamine pyrophosphate-deficient brain remain unknown. Therefore, the aim of our work was to find out how amprolium-induced thiamine pyrophosphate deficits (TD) affect distribution of acetyl-CoA in the compartment of pure cholinergic neuroblastoma SN56 cells originating from murine septum. Amprolium caused similar concentration-dependent decreases in thiamine pyrophosphate levels in nondifferentiated (NC) and differentiated (DC) cells cultured in low thiamine medium. In such conditions DC displayed significantly greater loss of viability than the NC ones, despite of lesser suppressions of PDHC activities and tetrazolium salt reduction rates in the former. On the other hand, intramitochondrial acetyl-CoA levels in DC were 73% lower than in NC, which explains their greater susceptibility to TD. Choline acetyltransferase activity and acetylcholine content in DC were two times higher than in NC. TD caused 50% decrease of cytoplasmic acetyl-CoA levels that correlated with losses of acetylcholine pool in DC but not in NC. These data indicate that particular sensitivity of DC to TD may result from relative shortage of acetyl-CoA due to its higher utilization in acetylcholine synthesis.

Phenotype-dependent susceptibility of cholinergic neuroblastoma cells to neurotoxic inputs.

A preferential loss of brain cholinergic neurons in the course of Alzheimer's disease and other encephalopathies is accompanied by a proportional impairment of acetyl-CoA synthesizing capacity in affected brains. Particular susceptibility of cholinergic neurons to neurodegeneration might results from insufficient supply of acetyl-CoA for energy production and acetylcholine synthesis in these conditions. Exposure of SN56 cholinergic neuroblastoma cells to dibutyryl cAMP and retinoic acid for 3 days caused their morphologic differentiation along with the increase in choline acetyltransferase activity, acetylcholine content and release, calcium content, and the expression of p75 neurotrophin receptors. Acetyl-CoA content correlated inversely with choline acetyltransferase activity in different lines of SN56 cells. In differentiated cells, aluminum (1 mM), amyloid beta(25-35) (0.001 mM), and sodium nitroprusside (1 mM), caused much greater decrease of pyruvate dehydrogenase and choline acetyltransferase activities and cell viability than in nondifferentiated ones. Aluminum (1 mM) aggravated suppressory effects of amyloid beta on choline acetyltransferase and pyruvate dehydrogenase activities and viability of differentiated cells. Similar additive inhibitory effects were observed upon combined exposure of differentiated cells to sodium nitroprusside and amyloid beta(25-35). None or much smaller suppressory effects of these neurotoxins were observed in nondifferentiated cells. Increase in the fraction of nonviable differentiated cells positively correlated with losses of choline acetyltransferase, pyruvate dehydrogenase activities, and cytoplasmic cytochrome c content in different neurotoxic conditions. These data indicate that highly differentiated cholinergic neurons may be more susceptible to aluminum and other neurotoxins than the nondifferentiated ones due to relative shortage of acetyl-CoA, increased content of Ca(2+), and expression of p75 receptors, yielding increase in cytoplasmic cytochrome c and subsequently grater rate of death of the former ones.