Nitric oxide has dual opposite roles during early and late phases of apoptosis in cerebellar granule neurons
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The involvement and the role of nitric oxide (NO) as a signaling molecule in the course of neuronal apoptosis, whether unique or modulated during the progression of the apoptotic program, has been investigated in a cellular system consisting of cerebellar granule cells (CGCs) where apoptosis can be induced by lowering extracellular potassium. Several parameters involved in NO signaling pathway, such as NO production, neuronal nitric oxide synthase (nNOS) expression, and cyclic GMP (cGMP) production were examined in the presence or absence of different inhibitors. We provide evidence that nitric oxide has dual and opposite effects depending on time after induction of apoptosis. In an early phase, up to 3 h of apoptosis, nitric oxide supports survival of CGCs through a cGMP-dependent mechanism. After 3 h, nNOS expression and activity decreased resulting in shut down of NO and cGMP production. Residual NO then contributes to the apoptotic process by reacting with rising superoxide anions leading to peroxynitrite production and protein inactivation. We conclude that whilst NO over-production protects neurons from death in the early phase of neuronal damage, its subsequent reduction may contribute to neuronal degeneration and ultimate cell death.
KeywordsCerebellar granule cells Apoptosis Nitric oxide Nitric oxide synthase Peroxynitrite Cyclic GMP
The authors wish to thank Prof. Shawn Doonan for his critical reading of the manuscript and Mr. V. Petragallo for the excellent technical support. This work was partially financed by: FIRB RBNE01ZK8F_003 to AA; CNR-MIUR Fondo FISR 16/10/2000 to A.A.; MIUR Contributi straordinari di ricerca/aree obiettivo 1 to E.M.
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