Characterization Of A Conformation-Restricted Amyloid Beta Peptide And Immunoreactivity Of Its Antibody In Human Ad Brain

Characterization of amyloid beta (A beta) oligomers, the transition species present prior to the formation of A beta fibrils and that have cytotoxicity, has become one of the major topics in the investigations of Alzheimer's disease (AD) pathogenesis. However, studying pathophysiological properties of A beta oligomers is challenging due to the instability of these protein complexes in vitro. Here, we report that conformation-restricted A beta 42 with an intramolecular disulfide bond at positions 17 and 28 (SS-A beta 42) formed stable A beta oligomers in vitro. Thioflavin T binding assays, nondenaturing gel electrophoresis, and morphological analyses revealed that SS-A beta 42 maintained oligomeric structure, whereas wild-type A beta 42 and the highly aggregative A beta 42 mutant with E22P substitution (E22P-A beta 42) formed A beta fibrils. In agreement with these observations, SS-A beta 42 was more cytotoxic compared to the wild-type and E22P-A beta 42 in cell cultures. Furthermore, we developed a monoclonal antibody, designated TxCo-1, using the toxic conformation of SS-A beta 42 as immunogen. X-ray crystallography of the TxCo-1/SS-A beta 42 complex, enzyme immunoassay, and immunohistochemical studies confirmed the recognition site and specificity of TxCo-1 to SS-A beta 42. Immunohistochemistry with TxCo-1 antibody identified structures resembling senile plaques and vascular A beta in brain samples of AD subjects. However, TxCo-1 immunoreactivity did not colocalize extensively with A beta plaques identified with conventional A beta antibodies. Together, these findings indicate that A beta with a turn at positions 22 and 23, which is prone to form A beta oligomers, could show strong cytotoxicity and accumulated in brains of AD subjects. The SS-A beta 42 and TxCo-1 antibody should facilitate understanding of the pathological role of A beta with toxic conformation in AD.