ABSTRACT
Increasing evidence indicates that maintenance of neuronal homeostasis involves the activation of the cell cycle machinery in postmitotic neurons. Our recent findings suggest that cell cycle activation is essential for DNA damage-induced neuronal apoptosis. However, whether the cell division cycle also participates in DNA repair and survival of postmitotic, terminally differentiated neurons is unknown. Here, we tested the hypothesis that G(1) phase components contribute to the repair of DNA and are involved in the DNA damage response of postmitotic neurons. In cortical terminally differentiated neurons, treatment with subtoxic concentrations of hydrogen peroxide (H(2)O(2)) caused repairable DNA double strand breaks (DSBs) and the activation of G(1) components of the cell cycle machinery. Importantly, DNA repair was attenuated if cyclin-dependent kinases CDK4 and CDK6, essential elements of G(0) --> G(1) transition, were suppressed. Our data suggest that G(1) cell cycle components are involved in DNA repair and survival of postmitotic neurons.
Subject(s)
Cell Cycle/physiology , DNA Repair/physiology , Neurons/metabolism , Animals , Apoptosis/drug effects , Cell Cycle/drug effects , Cell Cycle/genetics , Cells, Cultured , Cyclin-Dependent Kinase 4/genetics , Cyclin-Dependent Kinase 4/metabolism , Cyclin-Dependent Kinase 6/genetics , Cyclin-Dependent Kinase 6/metabolism , DNA Breaks, Double-Stranded/drug effects , DNA Damage , DNA Repair/drug effects , DNA Repair/genetics , Dose-Response Relationship, Drug , Flow Cytometry , Fluorescent Antibody Technique , G1 Phase/drug effects , G1 Phase/genetics , G1 Phase/physiology , Gene Expression Regulation/drug effects , Histones/metabolism , Hydrogen Peroxide/pharmacology , Immunoblotting , Immunoprecipitation , Mice , Mice, Inbred C57BL , Neurons/cytology , Neurons/drug effects , Phosphorylation/drug effects , RNA, Small Interfering/genetics , Rats , Rats, Sprague-DawleyABSTRACT
Loss of function of oncogenes, tumor suppressor genes and DNA damage processing genes has been implicated in the development of many types of cancer, but for the vast majority of cases, there is no link to specific germ line mutations. In the last several years, heterozygosity leading to haploinsufficiency for proteins involved in DNA repair pathways was shown to play a role in genomic instability and carcinogenesis after DNA damage is induced. Because the effect of haploinsufficiency for one protein is relatively small, we hypothesize that predisposition to cancer could be a result of the additive effect of heterozygosity for two or more genes, critical for pathways that control DNA damage signaling, repair or apoptosis. To address this issue, primary mouse cells, haploinsufficient for one or two proteins, ATM and RAD9, related to the cellular response to DNA damage were examined. The results show that cells having low levels of both ATM and RAD9 proteins are more sensitive to transformation by radiation, have different DNA double-strand break repair dynamics and are less apoptotic when compared with wild-type controls or those cells haploinsufficient for only one of these proteins. Our conclusions are that under stress conditions, the efficiency and capacity for DNA repair mediated by the ATM/RAD9 cell signaling network depend on the abundance of both proteins and that, in general, DNA repair network efficiencies are genotype-dependent and can vary within a specific range.
