Alzheimer's amyloid-beta peptide inhibits sodium/calcium exchange measured in rat and human brain plasma membrane vesicles

Abstract

Na +/Ca 2+ exchange activity was measured by monitoring vesicular Ca 2+ content after incubation in buffers containing 45Ca 2+. When Na +-loaded vesicles were placed into Na +-free buffer, vesicular Ca 2+ content increased rapidly and reached a plateau after two to three minutes. Only preaggregated amyloid-beta 1–40 (A β 1–40) and A β 25–35 reduced vesicular Ca 2+ content. Both peptides produced a maximal reduction in Ca 2+ content of approximately 50%. The peptides reduced Ca 2+ content with similar potency and half maximal effects were seen at less than 10 μM for A β 25–35. Calcium-loaded vesicles mediate a rapid Ca 2+/Ca 2+ exchange, which also was inhibited by aggregated A β 25–35. Aggregated A β 25–35 did not affect the passive Ca 2+ permeability of the vesicles. Aggregated A β 25–35 reduced Ca 2+ content in plasma membrane vesicles isolated from normal and Alzheimer's disease frontal cortex with less potency but the same efficacy as seen in rat brain. Aggregated A β 25–35 did not produce nonspecific effects on vesicle morphology such as clumping or loss of intact vesicles. When placed in the buffer used to measure Ca 2+ content, Congo Red at molar ratios of less than one blocked the inhibitory effect of preaggregated A β 25–35. When added in equimolar concentrations to freshly dissolved and unaggregated A β 25–35, Congo Red also was effective at blocking the inhibitory effect on Ca 2+ content. In contrast, vitamin E (antioxidant) and N-tert-butyl- α-phenylnitrone (spin trapping agent) failed to block the inhibitory action of aggregated A β 25–35. The exact mechanisms of A β-induced neurotoxicity in cell culture has yet to be solved. Accumulation of free radicals play a necessary role, but disruptions of Ca 2+ homeostasis are also important. The data presented here are consistent with a proposed mechanism where aggregated A β peptides directly interact with hydrophobic surfaces of the exchanger protein and/or lipid bilayer and interfere with plasma membrane Ca 2+ transport.