Normal processing of the amyloid beta protein precursor (beta APP) results in secretion of a soluble 4-kilodalton protein essentially identical to the amyloid beta protein (A beta) that forms insoluble fibrillar deposits in Alzheimer's disease. Human neuroblastoma (M17) cells transfected with constructs expressing wild-type beta APP or the beta APP717 mutants linked to familial Alzheimer's disease were compared by (i) isolation of metabolically labeled 4-kilodalton A beta from conditioned medium, digestion with cyanogen bromide, and analysis of the carboxyl-terminal peptides released, or (ii) analysis of the A beta in conditioned medium with sandwich enzyme-linked immunosorbent assays that discriminate A beta 1-40 from the longer A beta 1-42. Both methods demonstrated that the 4-kilodalton A beta released from wild-type beta APP is primarily but not exclusively A beta 1-40. The beta APP717 mutations, which are located three residues carboxyl to A beta 43, consistently caused a 1.5- to 1.9-fold increase in the percentage of longer A beta generated. Long A beta (for example, A beta 1-42) forms insoluble amyloid fibrils more rapidly than A beta 1-40. Thus, the beta APP717 mutants may cause Alzheimer's disease because they secrete increased amounts of long A beta, thereby fostering amyloid deposition.

The 4-kilodalton (39 to 43 amino acids) amyloid beta protein (beta AP), which is deposited as amyloid in the brains of patients with Alzheimer's diseases, is derived from a large protein, the amyloid beta protein precursor (beta APP). Human mononuclear leukemic (K562) cells expressing a beta AP-bearing, carboxyl-terminal beta APP derivative released significant amounts of a soluble 4-kilodalton beta APP derivative essentially identical to the beta AP deposited in Alzheimer's disease. Human neuroblastoma (M17) cells transfected with constructs expressing full-length beta APP and M17 cells expressing only endogenous beta APP also released soluble 4-kilodalton beta AP, and a similar, if not identical, fragment was readily detected in cerebrospinal fluid from individuals with Alzheimer's disease and normal individuals. Thus cells normally produce and release soluble 4-kilodalton beta AP that is essentially identical to the 4-kilodalton beta AP deposited as insoluble amyloid fibrils in Alzheimer's disease.

The 4-kilodalton amyloid beta protein (A beta), which forms fibrillar deposits in Alzheimer's disease (AD), is derived from a large protein referred to as the amyloid beta protein precursor (beta APP). Human neuroblastoma (M17) cells transfected with constructs expressing wild-type beta APP or a mutant, beta APP delta NL, recently linked to familial AD were compared. After continuous metabolic labeling for 8 hours, cells expressing beta APP delta NL had five times more of an A beta-bearing, carboxyl terminal, beta APP derivative than cells expressing wild-type beta APP and they released six times more A beta into the medium. Thus this mutant beta APP may cause AD because its processing is altered in a way that releases increased amounts of A beta.

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