DEOXYNUCLEOSIDE INDUCES NEURONAL APOPTOSIS INDEPENDENT OF NEUROTROPHIC FACTORS

Postmitotic sympathetic neurons are known to undergo a programmed cell death (apoptosis) when they are deprived of nerve growth factor (NGF) or treated with arabinofuranosyl nucleoside antimetabolites. Here we report the existence of a biochemical mechanism for the induction of n euronal death by an endogenous nucleoside in the presence of NGF. In s upport of such a mechanism we show that 2-deoxyadenosine (dAdo) induce s apoptosis in chick embryonic sympathetic neurons supported in cultur e by NGF, excess K+, phorbol 12,13-dibutyrate, or forskolin. Neuronal death was related to a dramatic increase in the dATP content of sympat hetic neurons exposed to dAdo (34.96 +/- 5.98 versus 0.75 +/- 0.16 pmo l/mu g protein in untreated controls, n = 9), implicating dATP in the toxicity. Supportive evidence for a central role of dATP was gained by inhibition of kinases necessary for phosphorylation of dAdo. 5'-Iodot ubercidin in nanomolar concentrations completely and dose-dependently inhibited formation of dATP and also protected against toxicity of sub millimolar lar concentrations of dAdo in sympathetic neurons. Although some of these actions of dAdo were remarkably similar to those report ed for human lymphoid cells, several were uniquely different. For exam ple, [H-3]dAdo was not transported into neurons by the nucleoside tran sporter, and therefore inhibition of the transporter (dilazep, nitrobe nzylthioinosine) did not prevent neurotoxicity by dAdo. Precursors of pyrimidine synthesis (2'-deoxycytidine, uridine) or NAD(+) synthesis ( nicotinamide) were ineffective in protecting sympathetic neurons again st dAdo toxicity. Finally, inhibition of adenosine deaminase by deoxyc oformycin or erythro-9-(2-hydroxy-3-nonyl) adenine did not potentiate the toxic effects of dAdo. Our results provide evidence for the first time that neuronal cells are as susceptible to nucleoside lethality as human lymphocytes are, and provide a new model to study the salvage p athway of deoxyribonucleosides in controlling neuronal populations thr ough programmed cell death.