Studies of the mechanism underlying increased Na+/Ca2+ exchange activity in Alzheimer's disease brain

Abstract The Na + /Ca 2+ exchanger was characterized in plasma membrane vesicles derived from frozen hustmortem tissues. The frontal cortex, temporal cortex and cerebellum of control and Alzheimer's disease (AD) tissues were compared. Na + /Ca 2+ exchange activity was defined as the change in vesicular Ca 2+ content seen after Na + loaded vesicles were diluted into choline buffer. The time course of changes in Ca 2+ content after dilution was similar in all three regions of control brain. In AD brain, both frontal and temporal cortex vesicles showed elevated Ca 2+ content, most evident as an increased peak Ca 2+ content at 2 min. The AD cerebellar cortex time course was similar to control and did not show an elevated peak at 2 min. No differences were seen in the passive permeability to Ca 2+ when comparing plasma membrane vesicles prepared from control and AD brain. Vesicles from the frontal and temporal cortex of AD brain showed increases in the V max of the initial velocity of Ca 2+ uptake when compared to control brain, whereas, the cerebellum did not. There were no significant effects of AD on the K m for Ca 2+ activation of the initial velocity. Ca 2+ influx measured during the rise in vesicular Ca 2+ content was elevated in vesicles from AD temporal cortex when compared to control. Two known inhibitors (exchange inhibitory peptide and dichlorobenzamil) of the cardiac Na + /Ca 2+ exchanger inhibited the human brain exchanger equally well in control and AD vesicles. Increased Na + /Ca 2+ exchange activity was not due to astrocytic gliosis. An antibody to the cardiac exchanger was used to determine the molecular weight of the human brain Na + /Ca 2+ exchanger. The molecular weight determinations from Western blots showed identical molecular weights of 100–110 kDa in both AD and control vesicle preparations. These data are entirely consistent with the proposal that increased Na + /Ca 2+ exchange activity in AD reflects the properties of surviving neurons with elevated Na + /Ca 2+ × exchange capacity and lend support to the ‘Ca 2+ hypothesis of AD’.