Two distinct conformations of Abeta aggregates on the surface of living PC12 cells.

Abeta42 has been found to associate rapidly to neuronal cells and is the primary constituent of senile plaques. In this study we monitored the aggregation of Abeta42 with living PC12 cells. Using photobleaching Förster resonance energy transfer, we observed one set of aggregates that displayed colocalization and another that displayed energy transfer. Cell surface aggregates were found to become resistant to potassium iodide (KI)-induced quenching. Exposed Abeta42 regions were probed with three monoclonal antibodies directed against the N-terminus, an internal sequence, and the C-terminus of Abeta42. Two populations of aggregates were revealed: one that bound all three antibodies, and one that bound all but the C-terminus antibody. Of interest, using fluorescent recovery after photobleaching, we observed no Abeta42 exchange within either type of aggregate. These findings offer what we believe is new insight into the conformations of Abeta42 that accumulate on the surface of living cells. One conformation is incapable of energy transfer, is sensitive to KI, and binds C-terminus-specific antibodies. The other conformation increases in number over time, is capable of energy transfer, is quencher-resistant, and has a sequestered C-terminus. With further studies to characterize Abeta aggregation on live cells, the underlying mechanisms leading to Alzheimer's disease may be revealed.