Mouse brain synaptosomes accumulate copper-67 efficiently by two distinct processes independent of cellular prion protein.

The prion protein (PrPC) is a copper-binding, cell-surface protein that plays an essential role in the etiology of transmissible spongiform encephalopathies. Atomic absorption spectroscopy studies have established that synaptosomal copper content is reduced in PrPC-deficient mice as compared with wild-type (WT) or PrP- overexpressing mice. To address the question of whether this is the result of a loss of function of PrPC in copper transport across the plasma membrane at the synapse, we have used synaptosomes incubated with 67Cu as a model system to characterize the mechanism of copper accumulation in nerve terminals. Our results demonstrate that mouse brain synaptosomes accumulate copper efficiently by at least two distinct mechanisms. In the presence of 1 mM EDTA, copper was taken up via a saturable high-affinity process that was moderately susceptible to competition by high concentrations of NiCl2. Uptake characteristics were clearly different in the presence of 400 microM histidine, with the most noticeable dissimilarities being considerably elevated uptake rates and moderate competition by ZnCl2 rather than NiCl2. No significant differences in copper uptake capability between WT and PrPC-knockout synaptosomes were observed under any of the experimental conditions tested in this study. Furthermore, preincubation of synaptosomes with an antibody binding to the copper-binding repeat region of the prion protein had no effect on copper uptake either. Taken together, our data indicate that synaptosomal copper uptake is independent of PrPC.