Abstract 8: APE1 Upregulation Reduces Oxidative DNA Damage and Protects Hippocampal Neurons from Ischemic Injury

Abstract

Background and purpose: Apurinic/apyrimidinic endonuclease 1 (APE1) is a multifunctional enzyme that participates in base-excision repair of oxidative DNA damage and the redox activation of transcription factors. We tested the hypotheses that APE1 upregulation protects neuronal structure and function against transient global cerebral ischemia (tGCI). Methods: Transient GCI was induced for 12 min in male Sprague-Dawley rats with the 4-vessel occlusion method. Low-dose proton irradiation or transgenic overexpression was generated to upregulate APE1. Lentiviral vectors expressing shRNA against rat APE1 were stereotaxically infused in the dorsal hippocampus to knockdown APE1 expression. Golgi staining and immunofluorescence staining of synaptophysin-1 and PSD-95 were used to label dendritic spines and synapses. Field excitatory postsynaptic potentials in the Schaffer collateral pathway were measured to assess the hippocampal synaptic function. Morris water maze was performed 22-27 days after tGCI to assess the learning and memory deficits. Results: Upregulation of APE1 protected hippocampal CA1 neurons against tGCI-induced cell loss and reduced apurinic/apyrimidinic sites and DNA fragmentation. Conversely, APE1 knockdown attenuated irradiation-afforded protection. APE1 overexpression inhibited the DNA damage response, as evidenced by lower phospho-histone H2A (H2AX) and p53 upregulated modulator of apoptosis (PUMA) levels. APE1 overexpression also partially rescued CA1 dendritic spines after tGCI and attenuated the decrease in field excitatory postsynaptic potentials in the Schaffer collateral pathway. Presynaptic and postsynaptic markers were reduced following tGCI and this effect was also blunted in APE1 transgenics. The Morris water maze test revealed that APE1 protected against learning and memory deficits for at least 27 days post-injury. Animals expressing DNA repair-disabled mutant APE1 (D210A) exhibited more DNA damage than controls and were not protected against tGCI-induced cell loss. Conclusion: APE1 upregulation, either endogenously or through transgene overexpression, protects DNA, neuronal structures, synaptic function, and behavioral output from ischemic injury.