Glycines from the APP GXXXG/GXXXA Transmembrane Motifs Promote Formation of Pathogenic Aβ Oligomers in Cells

Alzheimer's disease (AD) is the most common neurodegenerative disorder characterized by progressive cognitive decline leading to dementia. The amyloid precursor protein (APP) is a ubiquitous type I transmembrane (TM) protein sequentially processed to generate the beta-amyloid peptide (A beta), the major constituent of senile plaques that are typical AD lesions. There is a growing body of evidence that soluble A beta oligomers correlate with clinical symptoms associated with the disease. The A beta sequence begins in the extracellular juxtamembrane region of APP and includes roughly half of the TM domain. This region contains GXXXG and GXXXA motifs, which are critical for both TM protein interactions and fibrillogenic properties of peptides derived from TM alpha-helices. Glycine-to-leucine mutations of these motifs were previously shown to affect APP processing and A beta production in cells. However, the detailed contribution of these motifs to APP dimerization, their relation to processing, and the conformational changes they can induce within A beta species remains undefined. Here, we describe highly resistant A beta 42 oligomers that are produced in cellular membrane compartments. They are formed in cells by processing of the APP amyloidogenic C-terminal fragment (C99), or by direct expression of a peptide corresponding to A beta 42, but not to A beta 40. By a point-mutation approach, we demonstrate that glycine-to-leucine mutations in the G(29)XXXG(33) and G(38)XXXA(42) motifs dramatically affect the A beta oligomerization process. G33 and G38 in these motifs are specifically involved in A beta oligomerization; the G33L mutation strongly promotes oligomerization, while G38L blocks it with a dominant effect on G33 residue modification. Finally, we report that the secreted A beta 42 oligomers display pathological properties consistent with their suggested role in AD, but do not induce toxicity in survival assays with neuronal cells. Exposure of neurons to these A beta 42 oligomers dramatically affects neuronal differentiation and, consequently, neuronal network maturation.