Molecular Connections between DNA Replication and Cell Death in β-Amyloid-Treated Neurons

Background: Ectopic cell cycle reactivation in neurons is associated with neuronal death in Alzheimer’s disease. In cultured rodent neurons, synthetic β-amyloid (Aβ) reproduces the neuronal cell cycle re-entry observed in the Alzheimer’s brain, and blockade of the cycle prevents Aβ-induced neurodegeneration. DNA polymerase-β, whose expression is induced by Aβ, is responsible for the DNA replication process that ultimately leads to neuronal death, but the molecular mechanism(s) link-ing DNA replication to neuronal apoptosis are presently unknown. Aim: To explore the role of a conserved checkpoint pathway started by DNA replication stress, name-ly the ATM-ATR/Claspin/Chk-1 pathway, in switching the neuronal response from DNA replication to apoptosis. Methods: Experiments were carried out in cultured rat cortical neurons challenged with toxic oligo-mers of Aβ protein. Results: Small inhibitory molecules of ATM/ATR kinase or Chk-1 amplified Aβ-induced neuronal DNA replication and apoptosis, as they were permissive to the DNA polymerase-β activity triggered by Aβ oligomers. Claspin, i.e., the adaptor protein between ATM/ATR kinase and the downstream Chk-1, was present on DNA replication forks of neurons early after Aβ challenge, and decreased at times coinciding with neuronal apoptosis. The caspase-3/7 inhibitor I maintained overtime the amount of Claspin loaded on DNA replication forks and, concomitantly, reduced neuronal apoptosis by hold-ing neurons in the S phase. Moreover, a short phosphopeptide mimicking the Chk-1-binding motif of Claspin was able to prevent Aβ-challenged neurons from entering apoptosis. Conclusion: We speculate that, in the Alzheimer’s brain, Claspin degradation by intervening factors may precipitate the death of neurons engaged into DNA replication.