Intracellular generation of amyloid beta-protein from amyloid beta-protein precursor fragment by direct cleavage with beta- and gamma-secretase.

Two amyloid beta protein precursor (beta APP) fragments involving Met and 103 amino acids of C-terminus of beta APP (delta NOR-beta) and its KM-NL substitution (delta NL-beta) were expressed in COS-7 cells to clarify the proteolytic cleavages to generate amyloid beta protein (A beta). The 4.5-kD protein, A beta with additional N-terminal amino acids, and 4-kD A beta were directly produced and released from 12.5-kD expression proteins without any production of 11.4-kD C-terminal fragment starting at N-terminus of A beta and 3-kD "p3" A beta derivative. Intracellular 4-kD A beta was also detected. The substitution of KM-NL of beta APP found in Swedish familial Alzheimer's disease (AD) promoted the production of intracellular A beta and its release with no increase in level of 11.4-kD C-terminal fragment. These results suggested the presence of a distinct pathway in which A beta is directly cleaved at both N- and C-termini from beta APP fragment intracellularly to release A beta. Since KM-NL substitution enhanced intracellular A beta generation, this pathway may be associated with amyloidogenesis in AD.