Analysis of heterogeneous beta A4 peptides in human cerebrospinal fluid and blood by a newly developed sensitive Western blot assay

The beta A4 peptide, a major component of senile plaques in Alzheimer's disease (AD) brain, has been found in cerebrospinal fluid (CSF) and blood of both AD patients and normal subjects, Although beta A4 1-40 is the major form produced by cell metabolism and found in CSF, recent observations suggest that the long-tailed beta A4 1-42 plays a more crucial role in AD pathogenesis. Here, we established new monoclonal antibodies against the C-terminal end of beta A4 1-40 and 1-42, and used them for the specific Western blot detection. After optimizing the assay conditions, these antibodies detected low picogram amount of beta A4, and both beta A4 1-40 and 1-42 levels in CSF could be determined by direct loading of the samples. flood levels of beta A4 1-40 and 1-42 were also determined by specific immunoprecipitation followed by Western blot detection. We found that CSF beta A4 1-42 level is lower in AD patients compared with non-demented controls, although there was a significant overlap between the groups. The level of beta A4 1-40 in CSF, and of beta A4 1-40 as well as beta A4 1-42 in plasma, were not different between AD patients and controls. Besides the 4-kDa full-length beta A4 band, we could also detect several N-terminal variants of beta A4 in CSF and plasma of both AD patients and controls. Two N-terminally truncated beta A4 species migrating at the position of 3.3 and 3.7 kDa were found in CSF, while 3.7- and 5-kDa forms were found in plasma. The relative abundance of these various species were considerably different in the CSF and plasma, suggesting that the cellular source and/or clearance of beta A4 is different in these two compartments.