Objectives: 

Amyloid beta oligomers are considered the most likely neurotoxin in Alzheimer's disease. We have shown that oligomers, but not fibrils, promote calcium influx and mitochondrial calcium overload in neurons, leading to cell death. However, the primary target of oligomers is controversial. Here we aimed to investigate the target by testing the contribution of different calcium entry pathways and the formation of synaptic connections to the rise in cytosolic calcium concentration induced by amyloid beta oligomers in hippocampal cell cultures.

Materials: 

Amyloid beta oligomers were prepared by different reported procedures and their formation was tested by western blot. Then, their effects on cytosolic calcium concentration in fura2-loaded hippocampal cells were tested by fluorescence imaging. The protocol inducing the most reliable results was chosen for further research. The effects of oligomers with and without selected antagonists were tested in hippocampal cells before and after formation of synaptic connections. Finally, the effects of oligomers were also tested in different cell lines bearing or lacking NMDA receptors.

Results: 

We found that amyloid beta oligomers induced rises in cytosolic calcium concentration in hippocampal neurons. These effects were very small before synaptic formation and increased dramatically after synaptic formation. After synaptic formation cytosolic calcium rises were prevented not only by NMDA receptor and amyloid channel antagonists but also by blockers of Na+ and Ca2+ channels involved in synaptic transmission.

Conclusions: 

We conclude that amyloid beta oligomers induce calcium entry by formation of amyloid channels but this small effect is amplified first by activation of NMDA receptors and then by synaptic connections.

This work was funded by DIGYCYT (BFU2009-08967) and Junta de Castilla y León, Spain (VA 270 A11-2).