Heteromers of amyloid precursor protein in cerebrospinal fluid.

ABSTRACT

BACKGROUND: Soluble fragments of the amyloid precursor protein (APP) generated by α- and ß-secretases, sAPPα and sAPPß, have been postulated as promising new cerebrospinal fluid (CSF) biomarkers for the clinical diagnosis of Alzheimer's disease (AD). However, the capacity of these soluble proteins to assemble has not been explored and could be relevant. Our aim is to characterize possible sAPP oligomers that could contribute to the quantification of sAPPα and sAPPß in CSF by ELISA, as well as to characterize the possible presence of soluble full-length APP (sAPPf). RESULTS: We employed co-immunoprecipitation, native polyacrylamide gel electrophoresis and ultracentrifugation in sucrose density gradients to characterize sAPP oligomers in CSF. We have characterized the presence of sAPPf in CSF from NDC and AD subjects and demonstrated that all forms, including sAPPα and sAPPß, are capable of assembling into heteromers, which differ from brain APP membrane-dimers. We measured sAPPf, sAPPα and sAPPß by ELISA in CSF samples from AD (n = 13) and non-disease subjects (NDC, n = 13) before and after immunoprecipitation with antibodies against the C-terminal APP or against sAPPß. We demonstrated that these sAPP heteromers participate in the quantification of sAPPα and sAPPß by ELISA. Immunoprecipitation with a C-terminal antibody to remove sAPPf reduced by ~30% the determinations of sAPPα and sAPPß by ELISA, whereas immunoprecipitation with an APPß antibody reduced by ~80% the determination of sAPPf and sAPPα. CONCLUSIONS: The presence of sAPPf and sAPP heteromers should be taken into consideration when exploring the levels of sAPPα and sAPPß as potential CSF biomarkers.