Intracellular Accumulation of Toxic Turn Amyloid-β is Associated with Endoplasmic Reticulum Stress in Alzheimer's Disease

Amyloid-beta protein (A beta) accumulates in the neurons of Alzheimer's disease (AD) patients at an early stage of the disease. Recently, we found that A beta with a toxic turn at positions 22 and 23 accumulates in neurons in AD brain. Here, we studied the accumulation of A beta, toxic turn A beta and high-molecular-weight A beta oligomers in presenilin 1 (PS1) gene-transfected SH-SY5Y cells as well as in the brains of 3xTg-AD mice and AD patients. Immunostaining revealed that accumulation of toxic turn A beta was promoted in G384A- and I143T-mutant PS1-transfected cells and further enhanced by co-transfection of cells with the A beta-precursor protein (A beta PP) gene. In contrast, accumulation of high-molecular-weight A beta oligomers was promoted in mutant PS1 cells but attenuated by co-transfection of cells with the A beta PP gene. Toxic turn A beta was detected in the neurons of 3xTg-AD mice aged 2 months, when the mice were cognitively unimpaired. In contrast, high-molecular-weight A beta oligomers were detected in the neurons of 7-month-old mice, when memory dysfunction is apparent. Furthermore, immunostaining and western blotting for Rab4, Rab6 and GRP78 revealed increased levels of these proteins in mutant PS1 cells and their accumulation in the neurons of 3xTg-AD mice. Remarkably, GRP78 immunoreactivity was increased at 2 months of age. Double-label immunostaining of AD brain revealed an apparent association between toxic turn A beta and GRP78, an endoplasmic reticulum (ER) stress marker. Intraneuronal accumulation of toxic turn A beta may be associated with ER stress in the brains of AD model mice and AD patients at an early stage.