Immunohistochemical detection of the delayed formation of nonfibrillar large amyloid‐β aggregates

The occurrence of senile plaques consisting of amyloid‐β protein (Aβ) is a major neuropathological hallmark of Alzheimer's disease ( AD ). We previously developed and characterized monoclonal antibodies 31‐2 and 75‐2 that specifically bind to nonfibrillar Aβ 1–42 aggregates with diameters of more than 220 and 50 nm, respectively. Here, we report the use of these antibodies to examine the aggregation of exogenous Aβ 1–42 in cultured rat hippocampal neurons. From 6 to 24 h after transfection of Aβ 1–42 , antibody 75‐2 immunolabeled almost all transfected neurons, whereas 31‐2‐positive cells were restricted to a part of the transfected neurons and gradually increased in number. Expression of the F19S/L34P‐mutant Aβ 1–42 , which showed less of a tendency to aggregate, resulted in clearly reduced immunoreactivity to both antibodies. We also immunohistochemically investigated the temporal cortices of patients with AD and found that 31‐2 preferentially labeled the cores of a subpopulation of large amyloid plaques. The relative number of 31‐2‐immunoreactive plaques was found to correlate with the Braak stages of neurofibrillary tangles, but not with that of amyloid plaques. These results suggest that 31‐2‐reactive Aβ aggregates develop with a delayed time course in cultured neurons and amyloid plaques of AD brains.