Misfolded protein oligomers induce an increase of intracellular Ca2+ causing an escalation of reactive oxidative species

Alzheimer's disease is characterized by the accumulation in the brain of the amyloid beta (A beta) peptide in the form of senile plaques. According to the amyloid hypothesis, the aggregation process of A beta also generates smaller soluble misfolded oligomers that contribute to disease progression. One of the mechanisms of A beta oligomer cytotoxicity is the aberrant interaction of these species with the phospholipid bilayer of cell membranes, with a consequent increase in cytosolic Ca2+ levels, flowing from the extracellular space, and production of reactive oxygen species (ROS). Here we investigated the relationship between the increase in Ca2+ and ROS levels immediately after the exposure to misfolded protein oligomers, asking whether they are simultaneous or instead one precedes the other. Using A beta(42)-derived diffusible ligands (ADDLs) and type A HypF-N model oligomers (OAs), we followed the kinetics of ROS production and Ca2+ influx in human neuroblastoma SH-SY5Y cells and rat primary cortical neurons in a variety of conditions. In all cases we found a faster increase of intracellular Ca2+ than ROS levels, and a lag phase in the latter process. A Ca2+-deprived cell medium prevented the increase of intracellular Ca2+ ions and abolished ROS production. By contrast, treatment with antioxidant agents prevented ROS formation, did not prevent the initial Ca2+ flux, but allowed the cells to react to the initial calcium dyshomeostasis, restoring later the normal levels of the ions. These results reveal a mechanism in which the entry of Ca2+ causes the production of ROS in cells challenged by aberrant protein oligomers.

Automated summary

Alzheimer's disease is characterized by the accumulation of amyloid beta (A beta) peptide in the brain, which leads to the formation of senile plaques. The aggregation process of A beta also generates smaller misfolded oligomers that contribute to disease progression. These oligomers interact with cell membranes, increasing intracellular Ca2+ levels and producing reactive oxygen species (ROS). Our study found that the increase in intracellular Ca2+ occurs faster than the increase in ROS levels after exposure to misfolded protein oligomers. Removing Ca2+ from the cell medium prevents the increase in intracellular Ca2+ and abolishes ROS production. Treating cells with antioxidant agents prevents ROS formation, but does not prevent the initial increase in Ca2+, allowing the cells to restore normal calcium levels. These findings suggest that Ca2+ influx triggers ROS production in cells challenged by aberrant protein oligomers.