The mechanism underlying plaque-independent neuronal death in Alzheimer disease (AD), which is probably responsible for early cognitive decline in AD patients, remains unclarified. Here, we show that a toxic soluble Abeta assembly (TAbeta) is formed in the presence of liposomes containing GM1 ganglioside more rapidly and to a greater extent from a hereditary variant-type ("Arctic") Abeta than from wild-type Abeta. TAbeta is also formed from soluble Abeta through incubation with natural neuronal membranes prepared from aged mouse brains in a GM1 ganglioside-dependent manner. An oligomer-specific antibody (anti-Oligo) significantly suppresses TAbeta toxicity. Biophysical and structural analyses by atomic force microscopy and size exclusion chromatography revealed that TAbeta is spherical with diameters of 10-20 nm and molecular masses of 200-300 kDa. TAbeta induces neuronal death, which is abrogated by the small interfering RNA-mediated knockdown of nerve growth factor receptors, including TrkA and p75 neurotrophin receptor. Our results suggest that soluble Abeta assemblies, such as TAbeta, can cause plaque-independent neuronal death that favorably occurs in nerve growth factor-dependent neurons in the cholinergic basal forebrain in AD.