Substitution of phenylalanine by proline at position 19 of amyloid beta peptide results in an increased production of amyloid beta peptides with alternative N-termini after protein kinase C stimulation

Amyloid beta peptide (A beta) is derived from the beta amyloid precursor protein (A beta PP) by proteolytic processing. Three proteolytic activities are involved in the processing of A beta PP. beta-secretase generates the N-terminus of A beta, gamma-secretase generates the C-terminus and alpha-secretase cleaves in the central region of the A beta domain thus preventing A beta generation. In addition to the major beta-secretase cleavage at aspartate 1 of the A beta domain, alternative cleavages occur both in vivo and in vitro, leading to the generation of A beta-like peptides with truncated or elongated N-termini. To determine if the N-terminal cleavage of alternative A beta-like molecules is mediated by the beta- or alpha-secretory pathway, we stimulated alpha-secretory processing by activation of protein kinase C with phorbol esters. For this study we used a mutagenized A beta PP cDNA replacing a critical phenylalanine with proline at position 690 of A beta PP770. This mutation is close to the alpha-secretase site and inhibits the normal alpha-secretase cleavage at Lys 16 resulting in the production of high amounts of A beta-like molecules and truncated forms of A beta PPs. Activation of protein kinase C (PKC) in cells expressing the proline mutation did not inhibit A beta production, but led to an increase of the production of A beta-like molecules. This indicates that the N-terminus of alternative A beta molecules can be produced by a proteolytic pathway which is activated after PKC stimulation.