Evidence that the 42- and 40-amino acid forms of amyloid β protein are generated from the β-amyloid precursor protein by different protease activities

Cerebral deposition of the amyloid β protein (Aβ) is an early and invariant feature of Alzheimer disease (AD). Whereas the 40-amino acid form of Aβ (Aβ40) accounts for ≈90% of all Aβ normally released from cells, it appears to contribute only to later phases of the pathology. In contrast, the longer more amyloidogenic 42-residue form (Aβ42), accounting for only ≈10% of secreted Aβ, is deposited in the earliest phase of AD and remains the major constituent of most amyloid plaques throughout the disease. Moreover, its levels have been shown to be increased in all known forms of early-onset familial AD. Thus, inhibition of Aβ42 production is a prime therapeutic goal. The same protease, γ-secretase, is assumed to generate the C termini of both Aβ40 and Aβ42. Herein, we analyze the effect of the compound MDL 28170, previously suggested to inhibit γ-secretase, on β-amyloid precursor protein processing. By immunoprecipitating conditioned medium of different cell lines with various Aβ40- and Aβ42-specific antibodies, we demonstrate a much stronger inhibition of the γ-secretase cleavage at residue 40 than of that at residue 42. These data suggest that different proteases generate the Aβ40 and Aβ42 C termini. Further, they raise the possibility of identifying compounds that do not interfere with general β-amyloid precursor protein metabolism, including Aβ40 production, but specifically block the generation of the pathogenic Aβ42 peptide.

Simvastatin strongly reduces levels of Alzheimer's disease β-amyloid peptides Aβ42 and Aβ40 in vitro and in vivo

Recent epidemiological studies show a strong reduction in the incidence of Alzheimer's disease in patients treated with cholesterol-lowering statins. Moreover, elevated Aβ42 levels and the ɛ4 allele of the lipid-carrier apolipoprotein E are regarded as risk factors for sporadic and familial Alzheimer's disease. Here we demonstrate that the widely used cholesterol-lowering drugs simvastatin and lovastatin reduce intracellular and extracellular levels of Aβ42 and Aβ40 peptides in primary cultures of hippocampal neurons and mixed cortical neurons. Likewise, guinea pigs treated with high doses of simvastatin showed a strong and reversible reduction of cerebral Aβ42 and Aβ40 levels in the cerebrospinal fluid and brain homogenate. These results suggest that lipids are playing an important role in the development of Alzheimer's disease. Lowered levels of Aβ42 may provide the mechanism for the observed reduced incidence of dementia in statin-treated patients and may open up avenues for therapeutic interventions.