The oligomerization of amyloid beta-protein begins intracellularly in cells derived from human brain

The progressive aggregation and deposition of amyloid beta-protein (A beta) in brain regions subserving memory and cognition is an early and invariant feature of Alzheimer's disease, the most common cause of cognitive failure in aged humans. Inhibiting A beta aggregation is therapeutically attractiv e because this process is believed to be an exclusively pathological event. Whereas many studies have examined the aggregation of synthetic A beta pep tides under nonphysiological conditions and concentrations, we have detecte d and characterized the oligomerization of naturally secreted A beta at nan omolar levels in cultures of APP-expressing CHO cells [Podlisny, M. B., Ost aszewski, B. L., Squazzo, S. L., Koo, E. H., Rydell, R. E., Teplow, D. B., and Selkoe, D. J. (1995) J. Biol. Chem. 270, 9564-9570 (1); Podlisny, M. B. , Walsh, D. M., Amarante, P., Ostaszewski, B. L., Stimson, E. R., Maggio, J . E., Teplow, D. B., and Selkoe, D. J. (1998) Biochemistry 37, 3602-3611 (2 )]. To determine whether similar species occur in vivo, we probed samples o f human cerebrospinal fluid (CSF) and detected SDS-stable dimers of A beta in some subjects. Incubation of CSF or of CHO conditioned medium at 37 degr ees C did not lead to new oligomer formation. This inability to induce olig omers extracellularly as well as the detection of oligomers in cell medium very early during the course of pulse-chase experiments suggested that natu ral A beta oligomers might first form intracellularly. We therefore searche d for and detected intracellular A beta oligomers, principally dimers, in p rimary human neurons and in neuronal and nonneural cell lines. These dimers arose intracellularly rather than being derived from the medium by reuptak e. The dimers were particularly detectable in neural cells: the ratio of in tracellular to extracellular oligomers was much higher in brain-derived tha n nonbrain cells. We conclude that the pathogenically critical process of A beta oligomerization begins intraneuronally.