Heteromers of amyloid precursor protein in cerebrospinal fluid

Background: Soluble fragments of the amyloid precursor protein (APP) generated by alpha- and beta-secretases, sAPP alpha and sAPP beta, 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 sAPP alpha and sAPP beta 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 sAPP alpha and sAPP beta, are capable of assembling into heteromers, which differ from brain APP membrane-dimers. We measured sAPPf, sAPP alpha and sAPP beta 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 sAPP alpha. We demonstrated that these sAPP heteromers participate in the quantification of sAPP alpha and sAPP beta by ELISA. Immunoprecipitation with a C-terminal antibody to remove sAPPf reduced by similar to 30% the determinations of sAPP alpha and sAPP beta by ELISA, whereas immunoprecipitation with an APP beta antibody reduced by similar to 80% the determination of sAPPf and sAPP alpha. Conclusions: The presence of sAPPf and sAPP heteromers should be taken into consideration when exploring the levels of sAPP alpha and sAPP beta as potential CSF biomarkers.