Secreted Amyloid β-Proteins in a Cell Culture Model Include N-Terminally Extended Peptides That Impair Synaptic Plasticity

Evidence for a central role of amyloid beta-protein (A beta) in the genesis of Alzheimer's disease (AD) has led to advanced human trials of A beta-lowering agents. The amyloid hypothesis of AD postulates deleterious effects of small, soluble forms of A beta on synaptic form and function. Because selectively targeting synaptotoxic forms of soluble A beta could be therapeutically advantageous, it is important to understand the full range of soluble A beta derivatives. We previously described a Chinese hamster ovary (CHO) cell line (7PA2 cells) that stably expresses mutant human amyloid precursor protein (APP). Here, we extend this work by purifying an sodium dodecyl sulfate (SDS)-stable, similar to 8 Kda A beta species from the 7PA2 medium. Mass spectrometry confirmed its identity as a noncovalently bonded A beta 40 homodimer that impaired hippocampal long-term potentiation (LTP) in vivo. We further report the detection of A beta-containing fragments of APP in the 7PA2 medium that extend N-terminal from Asp1 of A beta. These N-terminally extended A beta-containing monomeric fragments are distinct from soluble A beta oligomers formed from A beta-40/42 monomers and are bioactive synaptotoxins secreted by 7PA2 cells. Importantly, decreasing beta-secretase processing of APP elevated these alternative synaptotoxic APP fragments. We conclude that certain synaptotoxic A beta-containing species can arise from APP processing events N-terminal to the classical beta-secretase cleavage site.