title: Heteromers of amyloid precursor protein in cerebrospinal fluid.
abstract: BACKGROUND
  Soluble fragments of the amyloid precursor protein (APP) generated by α- and β-secretases, sAPPα and sAPPβ, 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 sAPPβ 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 sAPPβ, are capable of assembling into heteromers, which differ from brain APP membrane-dimers. We measured sAPPf, sAPPα and sAPPβ 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 sAPPβ. We demonstrated that these sAPP heteromers participate in the quantification of sAPPα and sAPPβ by ELISA. Immunoprecipitation with a C-terminal antibody to remove sAPPf reduced by ~30% the determinations of sAPPα and sAPPβ by ELISA, whereas immunoprecipitation with an APPβ 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 sAPPβ as potential CSF biomarkers.
authors.forename: Inmaculada
authors.initials: I
authors.lastname: Cuchillo-Ibañez
authors.forename: Inmaculada
authors.initials: I
authors.lastname: Lopez-Font
authors.forename: Alba
authors.initials: A
authors.lastname: Boix-Amorós
authors.forename: Gunnar
authors.initials: G
authors.lastname: Brinkmalm
authors.forename: Kaj
authors.initials: K
authors.lastname: Blennow
authors.forename: Jose-Luis
authors.initials: JL
authors.lastname: Molinuevo
authors.affiliation: Instituto de Neurociencias de Alicante, Universidad Miguel Hernández-CSIC, Av, Ramón y Cajal s/n, Sant Joan d'Alacant, Spain. j.saez@umh.es.
authors.forename: Javier
authors.initials: J
authors.lastname: Sáez-Valero
mesh_terms: D000544:Alzheimer Disease / Q000134:cerebrospinal fluid / Q000175:diagnosis*; D053829:Amyloid Precursor Protein Secretases / Q000134:cerebrospinal fluid / Q000378:metabolism; D016229:Amyloid beta-Peptides / Q000134:cerebrospinal fluid / Q000378:metabolism; D016564:Amyloid beta-Protein Precursor / Q000134:cerebrospinal fluid* / Q000378:metabolism; D015415:Biomarkers / Q000134:cerebrospinal fluid; D002478:Cells, Cultured; D006801:Humans; D047468:Immunoprecipitation / Q000379:methods; D016875:tau Proteins / Q000134:cerebrospinal fluid / Q000378:metabolism
chemical_list: D016229:Amyloid beta-Peptides; D016564:Amyloid beta-Protein Precursor; D015415:Biomarkers; D016875:tau Proteins; D053829:Amyloid Precursor Protein Secretases