CHRONIC COPPER EXPOSURE CAUSES SPATIAL MEMORY IMPAIRMENT, SELECTIVE LOSS OF HIPPOCAMPAL SYNAPTIC PROTEINS, AND ACTIVATION OF PKR/EIF2 α PATHWAY IN MICE

Abstract

Copper is an essential element for human growth and development; however, excessive intake of copper could contribute to neurotoxicity. The effects of chronic copper exposure on spatial memory and the possible mechanisms remain unclear. Inductively coupled plasma mass spectrometry (ICP-MS) was used to measure the concentrations of copper in the serum and hippocampus of mice. Morris water maze test was performed to evaluate spatial learning and memory. Immunohistochemical staining and Western-blot analysis were performed to measure the changes of oxidative stress markers, synaptic proteins. TUNEL staining was performed to measure the change of apoptosis. Here we show that chronic exposure to copper in drinking water impaired spatial memory with simultaneous selective loss of hippocampal pre-synaptic protein synapsin 1, and post-synaptic density protein (PSD)-93/95 in mice. Copper exposure was shown to elevate the levels of nitrotyrosine and 8-hydroxydeoxyguanosine (8-OHdG) in hippocampus, two markers of oxidative stress. Concurrently, we also found that copper exposure activated double stranded RNA-dependent protein kinase (PKR) as evidenced by increased ratio of phosphorylated PKR at Thr451 and total PKR and increased the phosphorylation of its downstream signaling molecule eukaryotic initiation factor 2α (eIF2α) at Ser51 in hippocampus. Consistent with activation of PKR/eIF2α signaling pathway which was shown to mediate synaptic deficit and cognitive impairment, the levels of activating transcription factor 4 (ATF-4), a downstream signaling molecule of eIF2α and a repressor of CREB-mediated gene expression, were significantly increased, while the activity of cAMP response elements binding protein (CREB) was inactivated as suggested by decreased phosphorylation of CREB at Ser133 by copper exposure. In addition, the expression of the pro-apoptotic target molecule C/EBP homology protein (CHOP) of ATF-4 was up-regulated and hippocampal neuronal apopotosis was induced by copper exposure. Taken together, we propose that chronic copper exposure might cause spatial memory impairment, selective loss of synaptic proteins and neuronal apoptosis through the mechanisms involving activation of PKR/eIF2α signaling pathway. This work was supported by NSFC (81102154), Medical Scientific Research Foundation of Guangdong Province (B2012322, A2013598) and Shenzhen Scheme of Science and Technology (Medicine and Health) (201202086, 201302148).