Proteolytic antibody light chains alter beta-amyloid aggregation and prevent cytotoxicity

Beta-amyloid (Abeta), a peptide generated by proteolytic cleavage of the amyloid precursor protein (APP), is a major constituent of the neuritic plaques associated with Alzheimer's disease (AD). Up-regulation of alpha-secretase, which can hydrolyze Abeta between Lys16 and Leu17, has been proposed as a potential therapeutic strategy in the treatment of AD. Previously, we identified two light-chain antibody fragments that had proteolytic activity against Abeta, one with alpha-secretase-like activity and one with carboxypeptidase-like activity. Here we show that cleavage of Abeta40 by hk14, the light-chain antibody having carboxypeptidase-like activity, alters aggregation of Abeta and neutralizes any cytotoxic effects of the peptide. Cleavage of Abeta40 with c23.5, the light chain having alpha-secretase-like cleavage, substantially increases the aggregation rate of Abeta; however, it does not show any corresponding increase in cytotoxicity. The increase in aggregation resulting from hydrolysis by c23.5 can be attributed to the decreased solubility of the hydrolyzed products relative to the parent Abeta40, primarily the Abeta17-40 fragment. These results demonstrate that antibody fragment mediated proteolytic degradation of Abeta peptide can be a potential therapeutic route to control Abeta aggregation and toxicity in vivo. Our results also suggest that increasing alpha-secretase activity as a therapeutic route must be approached with some caution because this can lead to a substantial increase in aggregation.